Overview

The SL-C has benefited from a constant stream of development.  Some of the key improvements are shown below.  The sheer number of improvements indicates the seriousness by which Superlite sees the SL-C.  It has probably received the most development attention of any modern component car, anywhere.

It should be noted that since this is a user-created wiki without formal support from the factory, that there are actually many more improvements in the car- the list below represents only those we actually know about.

An '*' indicates an option.

Change  Description 
Brembo Brakes 

The 6-piston Wilwood racing-derived brakes were replaced with Brembo 4-piston brakes from the fifth-gen Camaro SS (and shared with exotics like Ferrari). The Brembos have dust seals and are OEM for street cars. Therefore they are legal in all jurisdictions, whereas the original Wilwoods were sometimes not due to their racing focus. The Brembos also have larger rotors, which required a change in the uprights. You can't change from the Wilwoods to the Brembos without also switching uprights as well. 

Pedals and ETC Bracket Early cars had a custom set of pedals welded up in stainless steel.  There were great pedals, but they were expensive to manufacture, and heavy.  New cars come with a superior 3-pedal set from TIlton that is manufactured from forged aluminum, has better bearings, and castings for a new electronic throttle control (ETC) bracket that is also now shipped with the kit.  This improved pedal assembly has removable foot pads that can be arranged to achieve different fitment, and pedal ratios to get just the right balance of effort and force.  The pads can be combined with custom-engraved pedal covers, as one builder has done.
Adjustable Pedal Mount* All cars can now be ordered with an adjustable pedal mount kit that allows the pedal assembly to be moved forward and backward for improved fit.  This is especially useful for cars that may have drivers with significant differences in leg length.  The kit consists of parts from a sand rail-type adjustable set, along with a SL-C-specific upper plate that combines the adjustable pan to the SL-C pedal set.  THe default setup with this kit requires the pedals to be adjusted by hand, while not driving.  At least one builder has used a linear actuator to move the pedals fore and aft with good success.
Emergency Brake* The SL-C is available with an emergency/parking brake for the standard Brembo brakes.  The kit consists of specific mechanical calipers, lines, custom brackets, pads, and a handbrake lever assembly for mounting in the cockpit.  Street cars typically are required to have some sort of handbrake, and this kit makes it easy to meet that requirement.
Carbon Ceramic Brakes* Optional carbon ceramic brakes were made available as an option or upgrade. They use parts from the Corvette ZR1.  The upgrade requires new uprights, machined specifically for the dimensional differences of the CC brakes.  These brakes are the ultimate for a street or track SL-C, with huge rotors and incredible stopping power.  The availability of these brakes puts the SL-C in an exclusive position in the component car industry.  These are very expensive, however, with the GM parts alone listing for around $17,000 (but available from select dealers for under $10K), in addition to the cost of the parts required from Superlite to mount them.
Windshield Wiper The original windshield wiper was adapted from a Cobra-style wiper as seen on the GT40s. It was replaced with a marine unit with two speeds, automatic park, and a seamless connection to the InfinityWire system. 
Instrument Package

The all-digital DigiDash  instrument package was replaced with a Koso RX2N. The Koso gauge has a large, 10,000 RPM analog tach and digital display of other functions, including speed. It comes with a specific fuel tank sender that is solid state and is specifically calibrated for the tank shape and gauge requirements, so unlike generic senders with other gauge packages that assume a square tank, this one has a curve that is modeled on the actual tank profile.  The Koso gauge has lights for turn signals, high beam, oil pressure and even an OBD2 warning light.

 

The DigiDash can still be used in place of the current gauge, but the fuel sensor will not be as accurate.  If you want to use the older Digidash, it is still available (though not now from the factory) and can be obtained in two versions, one of which includes GPS data logging, etc.

InfinityBox wiring system The traditional wire harness was swapped out to a modern power distribution system from InfinityBox.  The kit was expanded to include a complete mostly plug-and-play wiring harness for the SL-C. This was a radical change for the industry, as virtually all other similar cars used a traditional harness that was difficult to install, or a hacked-up donor car harness.  For more information check out the InfinityBox page here: http://infinitybox.com/
Street Tail* The original SL-C had a rear treatment that closely followed the GTP style cars, with a large open area in the back, an aggressive diffuser, and a high-mounted external wing.  A new 'Street Tail' is now available  with integral spoiler, large ducts to channel air into a builder-provided engine air intake, and a molded-in diffuser, as well as an explicit license plate area, and a car logo molded into the rear. Both tails share the same tail lights. The street tail doesn't come with a wing, but one can be added at extra cost. The spoiler on the street tail is there to add some downforce in those cases where a wing is not desired, or is prohibited by local law.  The street tail  is a no-cost option at order time.  About 70% of new SL-Cs are ordered with the street tail. 
LeMans* A Superlite "LeMans" body style option was developed. It's basically an open-top version of the SL-C, with specific rear clip, different doors, a different roll hoop, and a cut-down windscreen.  It's not intended for anything but open motoring, so a top of any kind isn't on the development horizon.  A couple have been ordered for racing, with a full cage, as well.
CNC Billet Arm box The welded lower control arm boxes inside the footwells were replaced by improved boxes CNC machined from billet.
Dropped Floors* Dropped floors were added as a no-cost option. They are selected by almost every buyer.  
Extended footbox* The SL-C already has very long footboxes.  But for a nominal fee, the footboxes can be extended such that the pedals can be mounted even farther forward in the car for increased leg room.  If you are over 6' 4" tall, this option may be interesting to you.  Shorter drivers don't need this. Applicable only to the drivers side of the car.
Short Master Cylinders Now standard on the SL-C are shorter master cylinders (compared to the originals).  Depending on how you mount the ETC, these shorter masters can offer up another 2+" of leg room.  Even more can be obtained with the extended footbox option above.
Chassis All panels that make up the chassis are now waterjet cut to provide improved accuracy in jigging, with cutouts (tabs and slots) that lock the panels into the proper position to improve fitment and chassis accuracy.  The Superlite SL-C chassis, in addition to being the stiffest on the market, is also the most square, as a result of these and other improvements.
Right Hand Drive Chassis* The car can be ordered in either right hand drive, or left hand drive.  Kit differences are a reverse-image dashboard, a different steering rack, and the center chassis spine offset to the left instead of the right (as is normal for LHD cars).  This is an option, but at present there is no additional charge for it.
Chassis The original chassis used 2X2s stacked on top of each other acting as the chassis spine.  These were changed to be a single 2X6" chassis tube.  Subtle changes in bracing were also implemented over time to improve stiffness, and allow for a greater variation in shifter mounting.
Chassis The 2X6 chassis member behind the seats was modified to allow greater seat recline for both the passenger and driver, compared to the original design.  This also cleans up the interior, as it offers fewer corners to cover with an interior (though this is obviated when the new interior tub is used).
Footbox  The footbox was narrowed slightly to improve body fit, as occasional thickness in any given body meant a tight fit in some cases, unless some excess FG was ground off.  The new footwell specification solves the potential fitment  problem caused by production variance in body thickness.  All chassis are made on a jig that ensures accuracy, and all are TIG-welded by certified weldors.
Rear Suspension The original rear suspension was designed with springs that went parallel to the axles.  This arrangement was changed early on to springs that used a different kind of rocker geometry to connecting the springs to the rear billet uprights.  This change improved access to the drivetrain, improved drivetrain clearance, and improved airflow over the shock/spring assembly for better cooling under race conditions.  The rear suspension rockers changed from a steel weldment to an all-billet construction for better appearance.
Rear Uprights Over time, the rear uprights have also been re-designed to improve stiffness at the toe link.
Lift Kit* Because the front end of the SL-C is very low, and long, an optional  hydraulic lift kit has been developed that lifts the front end up about 2" for increased clearance.  The kit consists of a hydraulic pump, two ram cylinders, a reservoir, and plumbing and wiring.  The lift action is pretty fast with the current pump, with the entire nose lifted up in a few seconds, and a drop occurring in less than a second.  The same kit can lift the entire car by adding two more ram cylinders to the rear suspension, though this implementation is rare, as the rear sits slightly higher anyway, and there is a very favorable departure angle in any case, especially with the street tail.  A key advantage is that the lift kit is completely integrated with the InfinityWire system, making the wiring portion of it completely plug and play.
Penske Shocks* For race cars, but applicable for aggressively-driven street cars, specially-valved Penske external reservoir shocks are available.  Superlite worked with Penske to develop a Superlite-specific version of their 8300 series shock that has an optimized valving for the car.  These are available on order, and can be retrofitted to all existing cars.
Race Suspension* For race cars,  a race-specific suspension was developed that allows fast at-track camber changes by just inserting or removing shims.  These race parts share an identical geometry with the street control arms, but have limited movement consistent with a race car, and have strong, encapsulated joints to tolerate abuse on the track.
Adjustable Motor Mount Original cars had to be ordered with a specific engine/transaxle combo in advance, so the engine mounts could be custom welded to the chassis.  This bespoke approach was changed to a steel, adjustable motor mount that allows a very wide variety of engines with a single part number, and also allows different engines to be used later if the owner changes direction- with just an adjustment of the new sliding motor mount.
Hinging

There have been improvements in materials in hinging and the way that the doors are mounted.  The original top door hinges were from a production car but better ones are now supplied in billet, and the lower hinges were formerly supplied as a steel weldment but have been provided in billet aluminum for some time now.

 

In addition, hinge systems for the rear clam have been improved.

Rear Hub Bearings The rear hubs were upgraded to domestic bearings of higher quality.  Even higher quality bearings are available for race use.
Fuel Tank The fuel tank was re-designed several times, the last iteration being larger, and less costly to manufacture, as well as being more leak-resistant due to fewer welds.  The new tank also has a custom welded flange for the Koso gas tank sender bolt pattern, so you don't have to drill and tap into a new tank.
Surge Tank An  aluminum surge tank is now included in the Stage 11 kit.
Radiator

The radiator has been modified over time, and it now has pass-through holes for the AC lines to go through.  This makes for a cleaner installation of the AC condenser in front of the radiator.

 

As of 2015, an improved radiator started to ship with the kit.  Except for a relocated hose position for the hot side, it is identical to previous versions.  Moving the hose to the top improves the coolant flow, and hence gives more efficient cooling.  This, combined with the improved fans that also started shipping in 2015, makes new kits much more efficient at cooling, offering cooling headroom for even the most radical engine combos.  These can be retrofitted to earlier cars for builders with extreme cooling requirements.

Air Conditioning Over the years, the standard air conditioning system has gone through several major changes in design and source.  The original system used an evaporator in front of the footbox.  The latest version, sourced from Vintage Air, but using a custom case for the SL-C, mounts the evaporator under the dash on the passenger side, inside the cabin.  This improves the AC performance dramatically over the initial version.  Special fiberglass ducts are included, to route conditioned air to the dash outlets, and the defroster.  
Front Dress for LS Engines* For the popular LS engine, Superlite has developed a front dress kit that routes the serpentine belt correctly, without the un-needed power steering reservoir, etc.  It consists of idlers and brackets and is available at a nominal cost.
AC Compressor Bracket There is a compressor bracket that fits the LS engine available from Superlite.  This bracket is adjustable, and uses a second, dedicated belt that runs on the back groove of the stock crank pulley.
Side Windows The conventional Lexan side windows were upgraded to ones that are hard-coated and are very scratch-resistant.  Unlike other cars where you are expected to "trim to fit", these windows are cut to precise size, and have polished edges, making them a quick bolt-on.
Wheels The original car offered standard wheels that were aftermarket replicas of Corvette C6 wheels.  These were later replaced with a choice of Forgestar wheels in several styles, at no extra cost.  The rotary-forged Forgestar wheels are stronger, lighter, fit over more brake combinations, and offer a wider range of styles to fit every taste.
Centerlock Wheels* Forgestar wheels are now available with centerlocks (instead of the normal 5-bolt GM pattern).  The same Forgestar blanks as the standard wheels are machined to specific Superlite offsets, and drilled with a large single center hole instead of the normal 5-bolt pattern.  Optional adapters allow these wheels to be bolted up to the standard uprights.
Race Wheels* For race use, wheels from CCW were made available that are extremely light, strong enough for race use, and available in specific offsets and sizes appropriate for the SL-C in race trim.  These wheels are available as large as 18X11 in the front, and 18X13 in the rear.  They are priced very competitively for their performance.
 Interior Tub* A optional two-piece option is available in fiberglass or carbon fiber, and provides a very finished look to the interior, as well as a narrower, more flowing console that is integrated to the rear bulkhead and roof portion of the tub.  In addition to the improvement in appearance, the tub is designed to lock the body in perfect alignment, so body placement on the car is made even simpler.
Track Splitter* and Race Splitter

In addition to the original flat splitter, an optional new, deeper model with a slightly increased radiator opening was released, mainly for track use, though many people run them on the street.

 

In addition to the Track splitter, a race-only splitter is also available. As seen on the #03 and #01 race cars, it is actually a fully-formed wing, and is part of a CFD-derived and track-tested front aerodynamic treatment. This splitter is way too deep to run on the street, and only works correctly with the rest of the aero package for the front.

Fender Vents* To improve front aero grip, fender vents were designed and manufactured.  These can be bonded in and finished, or bolted in for a race look, making replacement easier and faster.
Canards* Canards were developed for the front end, to improve front end grip.  These are typically made of aluminum.
Side-Exit Exhaust*

Attractive flanges of gloss carbon fiber were developed used to protect the body from the heat generated by the exhaust.  These were derived from the flanges used on the factory race car.

Carbon Fiber Wing* The original fiberglass wing as used in the race tail was supplemented with one in carbon fiber that also has a more aggressive chord for improved efficiency.  Both wings were later rationalized to have identical wing mount spacing to reduce the number of mounting options and SKUs.
Wing Mounts* The original wing mounts for the race tail were revised, the newer mounts being narrower, and mounting the wing higher and more rearward, improving downforce without an increase in drag.  For race applications, a "swan neck" wing mount is available that is manufactured in billet, and is designed to mount the wing from the top, as is done in modern GT cars for improved functionality (the disturbed airflow from the bracket to the wing occurs on the side of the wing that is least affected by turbulent airflow, so the wing is more efficient when mounted this way).
Roll Cage Finish The powder coat finish has been replaced by nickel-plating for better appearance, and longer life, since it is more scratch-resistant.  The rear suspension brace is also nickel plated.
Side-Impact Bars* For track cars, side-impact bars were developed to limit the damage of a side-impact hit on track.  These bolt into any car (without the optional padded door panels or interior tub) and offer a great race look as well as adding additional protection.
Race Cage* For all-out race cars, the factory is able to provide (when specified at car order time) a complete NASA, SCCA and FIA-legal cage.  This is a race-only option.
Delivery Another change to the car delivery process (though not the car itself) is that every shipped car has every part it is shipping with labeled with the customer's name and the date, and is photographed before packaging in the car for shipment.  Every photo, along with an order-specific ship/pack list is copied to a USB drive which is shipped to the customer with the car.  In this way, the customer (and Superlite) know exactly what parts were shipped, any backorders, and where the parts are.  There have been several instances where a customer thought there were missing parts, until the box in which they were packaged was identified in the photos in the USB drive. In addition to the shipping list and pictures, an electronic copy of the Build Manual is copied to the USB drive.
Track Suspension upgrade* While there has been a race-only suspension kit for some time, there is now a "middle" solution between a street car and a full-on race car.  Using the existing street control arms and uprights, there is a kit to replace the ball joints in the street arms with rod ends with spherical bearings.  This is an exciting option for track cars, as it makes the suspension much more durable.  However, the new bearings don't have the full movement of the ball joints they replace, and as a result, the turn radius as well as overall suspension travel is reduced.  This is a satisfactory compromise for track cars, but not recommended for the street.  These bearings still need to be lifed out, as do all wear items in a race car, but their life and performance is much improved over the original ball joints.
New Seats

The SL-C originally came with a single-layer fiberglass seat based on the iconic Porsche 962 endurance racer.  These seats are still available, but have been superseded by a new two-layer seat with integrated shoulder harness holes, and shoulder wings that are slightly narrower, making fit easier.   The new seats, like the old ones, can be ordered, at extra cost, in Kevlar or carbon fiber.  Originally an extra-cost option, the fiberglass version of the new seats is shipping with every new SL-C, though the original seats are still available as a no-cost swap at time of order.

 

The original seats are still the lightest available, and also the lowest.  It you are tall in the torso, and looking for the lowest seat possible, the original seats may be the best choice, even as you will need to slightly modify them for your selected harnesses (the seat does not have harness cutouts, as the exact placement will vary according to placement and driver size). 

Rear Window Vents*

All mid-engined cars generate heat over the engine.  Until recently, the only way to vent it in an SL-C was to make your own vents in the Lexan window, cut holes in the fiberglass normally cut out for the rear window, or possibly hook up some fans to pull hot air out of the engine compartment.  Now available are carbon-fiber vents that are designed to fit perfectly along the edges of the existing window opening.  These fit both the race tail and the street tail, and require you to modify the lexan window to fit.  They are a limited availability item as they are difficult to make, and are available to order now.  They look great, and really exhaust heat from the engine compartment, keeping everything cooler, and happier.  Initial pricing is $895 for the vents in carbon fiber, ready to install with automotive clear coat already applied, or in fiberglass (requires prep and paint to finish) at $595

Upgraded fans In early 2015, upgraded fans were introduced as a running production change. Some builders thought that the previous fans were inadequate, though with a good shroud, they were really not. In any case, these stronger, more powerful fans will aid in cooling the engine faster than the previous ones.  These are the same fans used on the factory race cars.
Upgraded Steering Wheel In late 2015, a new steering wheel became a standard part of the kit.  The previous wheel was replaced with a new leather/suede wheel of much higher quality and appearance. The new wheel is backwards-compatible with the old wheel, and is available from the factory as a replacement part.
Stronger stub axles In late 2015, the formerly optional, race-derived 300M stub axles became standard for all cars. These are incredibly strong, and are 1000HP capable. These are the same stub axles that the factory race SL-C ran, and that are in use in most SL-Cs that are seeing severe race duty.  You may want to trim the tip of the axle as it protrudes more than the original ones, or just use a hub/wheel cap that is dished.
Race-legal fuel cell* There is now an optional, custom-fit fuel cell that is shaped just like the standard tank.  It fits in the same place as the standard fuel tank, so it is easy to swap to it if a builder decides to convert a street car to a race car. Not required for most track day events, but definitely mandated for wheel-to-wheel racing by all sanctioning bodies including SCCA, NASA, etc. Adds more safety to a street car build as well (though the standard tank is in an extremely protected location).
Improved rear hinges for the street tail An improved set of hinges for the rear of the street tail now allow the rear clam to open much farther, making access easier, affording a better view of the engine compartment, and improving maintainability. This is a running change, and all production cars after January 2016 will have the hinge set. Existing customers may upgrade their cars by purchasing the new hinges from the factory if desired. 
Drilled and slotted rotors The original Brembo brakes came with an plain rotor. New cars from 2016 now have a set of 4 drilled and slotted rotors for a small decrease in weight, improved pad off-gassing, and better looks. These are available as a retrofit for older cars.