No, the AMG Performance Exhaust is standard on both the C63 and C63S as of the FL. Not sure where you are seeing it being optional on the S. The difference between the S and non-S are as follows:
- More power and torque
- Dynamic engine mounts
- Beefier brakes with two piece composite rotors and optional carbon ceramic brakes
- 9-stage traction control from the GT3 as found in the GT R, GT R Pro and GT BS (The S is worth it for this option alone)
- Race mode and AMG DYNAMICS Master mode
Those are the things you can't get in the non-S at all. On top of that the S has more standard features such as the AMG Drive Unit, Track Pace etc, and other options are cheaper than on the non-S, and some options are exclusive to the S. Not all interiors are available on the non-S for example. Because of that the price difference shrinks to something around $4000 once you load it up with equivalent options, and for those $4000 you get the above exclusive features on the S. Both the S and non-S now have the electronic locking differential, so that's no longer a differentiator as it was for the PFL. The main differentiator now is the 9-stage traction control, which is an awesome piece of kit, even if you never manually adjust it. It's just such a superior traction control system that doesn't kill the fun and instead helps and works with the driver.