One of the tasks in building an SL-C will be the exhaust system. This can be relatively simple, or quite complicated, depending on your requirements, and the packaging constraints of the drivetrain, and whether you have selected the race or street tail (either is available at order time at no cost). Recently, the factory has started to supply exhaust systems for those who can't or don't want to weld up their own.
In general, the street tail has much more room to run a rear-exit exhaust. The race tail fits tighter to the car, and because of the diffuser, there are fewer places to route the exhaust.
Side-exit exhaust systems can be used with either tail -- the exhaust pipes in these cases exit out the side of the body just ahead of the rear wheels-- but side exhaust systems tend to be implemented with very short pipes, making it difficult to package a muffler of adequate size, so these tend to be louder than a rear-exit exhaust, all things being equal. At least one builder has built an elaborate side-exhaust system with good muffling, but the system was complex and appeared to be expensive, though it was apparently quite effective.
Some builders make headers that are custom-fabricated for their car. These are relatively expensive, but offer great appearance, potentially more packaging options (though they can also make packaging harder if a certain primary length is required), and have the potential to make more power, or make it at a desired place in the powerband. Custom headers and exhausts can be made in race shops around the country. You'll normally have to drop off the car at the shop to have it done. Typical prices for these range from $1500 to $4500, depending on where they are done, the material chosen (stainless or inconel are much more expensive than mild steel), the size and quantity of mufflers and cats, and the packaging constraints in the particular car.
Factory LS7 headers
Many people use the factory LS7 exhaust headers. In fact, the factory 01 race car uses these, in part to show what a pretty stock engine can accomplish in an SL-C. According to GM engineering sources, these are good until about 670 HP, where headers can begin to make more power. They were also used in the mighty Corvette ZR-1.
The LS7 headers offer a very compact fit, good power handling capabilities, and are made of stainless steel, so resist corrosion. They are also relatively quiet, and use proven GM sealing techniques. They can be found on eBay for low cost (around $90 to $150 for the pair), as many C6 Z06 drivers are swapping them out for headers.
These headers use factory gaskets which seal well, but are relatively expensive. In addition, the non-standard outlet on the headers means that some fabrication will be required to mate the rest of the exhaust system to the headers. This is relatively easy, and now well-understood in the SL-C community. There are several sources for the LS7 header outlet flanges now available, including the Superlite factory. Another source is here.
The rest of the exhaust system consists of muffler(s), catalytic converters if needed, pipes, and possibly v-band connectors (to make assembly and disassembly easier).
Most builders are using 3" pipe everywhere. It normally doesn't make sense to use smaller pipe, as the restriction is much greater, even dropping down to 2.5" diameter pipe. Bigger pipe than the 3" recommended here doesn't do much more for flow, and for most engines in these cars, can actually reduce power. If your design is very tight, consider using oval tubing, but keep to the 3" size if possible.
Most builders fabricate their exhaust from mild steel, and typically coat it with a ceramic-based coating. This makes for good looks, and long life, as well as better heat retention in the system.
For very high-end systems, materials like stainless steel (use 304L or better) or inconel can be used. The primary benefit of these materials is that thinner pipe can be used, and thus weight can be dropped. But costs are higher, both for materials, and for labor. These materials also support higher temps, so turbo applications will normally use stainless, or inconel for their systems.
Those builders that select side exhausts can obtain nice carbon fiber bezels from the factory that make the side outlet look very finished.
A good exhaust system doesn't just happen- it has to be designed. There is a good article on exhaust science here.
And of course there is a wealth of information about exhaust system design on the web. Just exercise normal skepticism, and don't fall into the "if it's louder, or bigger, it must be making more power!" trap.
If you plan to register the car in a state where emission tests are required, you must design your system with that in mind. Most emission tests will immediately fail a late-model computer-controlled car without a catalytic converter, so plan accordingly.
In general, a rear-exit exhaust will be quieter, due to the longer pipe length and possibility of more and larger mufflers.
Bigger-cube engines, and those with even a slightly more radical cam will normally be louder, all things being equal, so plan accordingly. For example, the same exhaust on a cammed LS7 will be much louder than a stock LS3