I have the Dinan intake and exhaust. The interest is mainly aesthetics and a bit of sound. Certainly not power. My Dinan Stage III suspension is a lot more important to my enjoyment of the car than the exhaust.
I would point out that unless you are going catless, the M3 is really known more for it's mechanical growl under power, which turns into a real snarl at high revs. My point being is that most of the enjoyable noises from this vehicle come from the FRONT, rather than out the exhaust. If you really want noise from the exhaust, go catless with a crazy system, or buy a C63 or a Shelby Mustang GT500.