Other transaxles for the SL-C include the Albins (installed in at least one SL-C in Australia), and various race transaxles, as from Emco and Xtrac. There are other possibilities, including the CIMA range from Italy (used in some hypercars like the Konigsegg). At least one SL-C builder has fitted a Hewland transaxle for a track-focused car, as shown below:
Other interesting choices include transaxles from current Ferrari cars (especially the 360/430/458 series) but these suffer from very high cost, low availability and exotic parts prices. If you are willing to use previously owned units, possibilities include transaxles from wrecked mid engined cars like the Audi R8, certain Lamborghini's, and similar cars.
As used in the Saleen S7, the RBT transaxle is based on the venerable ZF that saw duty in the original GT40 (after the Colotti box was dropped for reliability reasons). It was also used, in slightly different form, in the BMW M1 and the Pantera. These are expensive, and generally not the optimum choice for the SL-C.
On the lower end of the scale, the Audi 01E series of transaxles have been used in mid-engined kit cars in the past, and CAV uses them as their default unit. These can be obtained with a wide range of gears, and factory and aftermarket limited slips are available. Many of the parts interchange between the Audi transaxles and the Porsche 944 series as well. There is a detailed discussion of the Audi transaxles here.
Racing transaxles can be acquired new directly from the manufacturers above. CIMA does not appear to have a domestic distributor. Some units may be available only from wrecking yards. Others are available from the manufacturers as shown.
Most of these other transaxles are not well documented to the public, so the knowledge base is fairly low, except for the racing versions, which can be serviced domestically (at high cost). Many of these transaxles do allow for paddle shifting, and this can reduce lap times considerably compared to a traditional H-pattern gearbox. Gearing for the racing transaxles is wide open, and can be specified at order time. Used transaxles from wrecked exotics tend to not have a wide range of gear ratios, so if you are considering this route, understand the existing ratios of the boxes you are contemplating.
All of these transaxles will need some sort of adapter to install them in the SL-C, and to the common engine choices. This can be avoided by selecting a complete OEM drivetrain (i.e., engine and transaxle) from a wrecked car, which also simplifies the ancillaries like clutches, starters, etc which are often a major problem when lashing up an engine to a transaxle for which it was never intended.
Most of the newer mid-engined transaxles are laden with electronic aids- as are the Ferrari units, for example. These tend to enhance the driving experience for most users, but come at a steep cost- specifically the complexity of adapting the transmission control units to vehicles to which they were never intended. This factor alone makes these units rare in cars like the SL-C.
Racing transaxles tend to have a relatively short life, perhaps less than 10,000 miles before they need to be rebuilt. Also, they are relatively noisy, sometimes extremely so, due to straight-cut gears. These factors, and their initial high cost has limited them to true race cars in the SL-C.
The RBT or ZF transaxles are known for their relatively poor shifting, and cannot be reliably speed shifted without risk of damage.
Complete drivetrains will have these already installed, otherwise you will need to adapt off the shelf units, or make custom ones as needed. At a minimum, expect to have a custom flywheel and clutch assembly for non-standard builds.
Most all of the transaxles will need some kind of custom or semi-custom linkage. The Ricardo can use the stock Ford GT shifter, which makes it a duplicate of the factory setup, retaining OEM quality feel and accuracy. Several different shifters are available for the Porsche series, the later models of which use cable shifters which are generally easier to install compared to a rod linkage.
Shifters from other production cars may also be used; some builders of other cars have used the Toyota MR2 and Ford Escort shifters with few modifications. Shift cables can be ordered in custom lengths, and with the the correct fittings on the ends in most cases. See CMS, M2 Cables and others for custom cables.