All cars have had axles provided by The Driveshaft Shop, with 930-style constant-velocity (CV) joints.
The 930 CVs are very strong, and there is a lot of info on the web about them, including rebuilding and maintenance. Since the axles come already assembled, you don't need to do anything but inspect them and install onto the stub axles on the car, and the drive flanges on the transaxle. All of the axles provided have been manufactured with 300M steel, making them extremely strong, and probably overkill for most powertrains, but this gives a great safety factor. It is very unlikely that axle problems will occur on an SL-C. Sometimes the 930 CVs don't match up to the output flanges on the transaxles, so adapters can be installed. These are standard on the Ricardo transaxle, and are generally not needed with Porsche transaxles. Torque for the Ricardo axles is 57 ft-lbs on the adapter and axle bolts.
It is crucial that you check the torque on axle bolts, including where they bolt to the transaxle and to the stub axles in the uprights. Use safety wire if possible, and use a paint mark to help reveal loosening over time (i.e., after proper torque, use a paint stick or steal your wife's nail polish and paint a small stripe over the bolt and the adjacent metal. If the bolt head moves, the stripe will be interrupted and the loosening will be obvious with just a glance).
The 930 CVs are designed to run acceptably at pretty high angles. Don't move the engine or transaxle in a misguided attempt to make the angles "perfect". In fact, the CVs actually wear longer if they have a small angle in them as they run.
For the record, sand rails also typically run these CVs because they are very strong- and they run driveshaft angles of up to 22 degrees. Don't worry about the angles in an SL-C they are much smaller than you need to worry about.
The axles can be made thicker, to withstand more power, but generally other parts will break first. Upgraded CVs from the Porsche 935 are available from The Driveshaft Shop if necessary. They can take your old axle assemblies and convert them. In general, you won't need upgraded axles unless you are drag racing, with big power.
If you are racing an SL-C with big power, you may want to upgrade the CVs to racing standards. Call the factory or DSS for info on upgrading your CVs to the same ones used on the IMSA Prototypes.