ac schnitzer do an axel back that will work with your exhaust. I had an X-Pipe in place of the centre resonator (as well as a secondary cat delete. A Remus centre pipe would be your best bet though as that just bolts on and then you have to clamp it on the rear! that way you'll have a complete section to fit come emission test time
http://www.remusuk.com/model.asp?id=...)%20M3&man=BMW
I wish I'd seen this before I had my exhaust done as it would have only cost a few hundred more and can easily put back to stock