The 328 and M3 have totally dual exhaust until it meets way back at the muffler. Unfortunately, the 323 bottlenecks the exhaust ahead of the catalytic converter and then runs a single pipe all the way back to the muffler. There are actually significant power gains to be had with a 323 if you upgrade to the 328 or M3 midpipe with a 328 or M3 cat-back. That's the route I would recommend for this application. As far as having separate dual mufflers, etc., I wouldn't do that. There really isn't room to do it easily due to the spare tire well and it looks silly almost all of the time when people try to do it. Almost universally looks ghetto rigged.
1998 BMW 328is Shooting Brake
1966 Pontiac GTO
2007 Subaru Impreza 2.5i 5-door
View my photos: Caught in the Wild