Fitted a Hamann Cat back stainless 4 exit system about 3 years ago. Best mod ever. Brought off ebay

General comments:
All stainless so last forever
Looks way better with 4 tailpipes (2 was standard 3.0i sport)
Way better sound, but beware they can wake small children when starting them up...silent at cruise speed
Smaller boxes and lighter than standard

Forgive me if stating the obvious but you can buy the different size exhaust infill plates from BMW for a few $ to fit any size exhaust. They are interchangeable.

A mod I'd go with

__________________
X5 3.0i Sport - Brembo rotors, Hamann Exhaust, custom Xenon & AE lighting, Retrofit Bluetooth
Z4 3.0i Roadster
CJ10a Jeep
TJ 'Challenge' Jeep