i believe the pipes on the 330i are 1 3/4" and the m3s are 1 1/4", you can buy a piece of pipe to reduce the size of the exhaust system and then the back box should be able to fit easily.
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i have 328 and was wondering wheater the 320 single pipe exhaust system would fit on it and what would be the out . because i was thinking more back pressure more power .
The headers make a bigger difference overall than the full exhaust.

What other mods have you done or are you planning?
what difference if it was a 323 single pipe which i guess would be a little bigger .
Not really touched it but lookin around at supercharger kits .
A single larger pipe wont make much difference IMHO. Overall size is the deciding factor. A 421 or 422 manifold is best for most applications.

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