Sounds like a great engineering project. Hopefully you have access to an M3 for measurements and photos? If not, contact your local BMW club or service shop and ask around. Since you are specifically targeting the M3, it would be best if you could design something that sort of looks like it would actually fit on the car.
Why a "full race exhaust"? The best race exhaust is probably just straight pipes -- pretty boring project
Why not set your design as a "flow and sound optimized street exhaust"? You could first model the factory exhaust and then compare flow (sound is more difficult) to your own design. The factory muffler has 2 roughly 90° bends for each pipe inside. It is claimed that these restrict flow. Do they? How much?
What CAD system do you use? Solidworks for instance has a nice flow modeling package that would be very useful for this but for your project, maybe it's better to calculate what you can on your own.
It seems that most street exhausts for the M3 that don't drone (unpleasant resonating noise heard in the cabin) in the 2k-3k RPM range use Helmholtz resonators -- that right there is a great engineering design exercise for your project (examples include: AA, Dinan, Corsa). Some good info. to get you thinking (click on the M3 exhaust link): http://www.dinancars.com/university/
You can also search the posts here and other sites for some good discussion on Helmholtz resonators in exhaust systems.