Found these on SportAuto. It's interesting to see how little modifications were needed for the M3 to be race ready. Of course, the GT4 is an ameteur series with an emphasis on reduced running costs, high reliability and the carry-over of production parts.
Some notable changes from the stock car:
1. Rear spring/shock configuration has been modified to a strut configuration.
2. AP Racing brakes on front and rear. Front seems like a monobloc, rear appears to be two-piece.
3. Carbon fiber fuel tank.
4. Akrapovic full exhaust.
5. Front brake cooling channels.
Also, I guess now we can appreciate why the Akrapovic full system we buy have catalytic convertors downstream of the stock car's first and second sets of O2 sensors. Looks like the Akrapovic was designed for the GT4 car, which requires cats in the system, but they are placed further downstream to improve performance?
BTW, BMW Motorsport (not to be confused with M GmbH who makes our road-going Ms) just announced a E89 Z4 GT3 race car w/ a 480 bhp V8! http://e89.zpost.com/forums/showthread.php?t=332955