Have you considered something like this? I like the large single oval tip but they do have to cut the factory one. The exhaust shop would be able to weld the factory axleback back on though when you get ready to trade. AWE is working on an exhaust suite but their parts are on the high side. You will get more performance gains with a downpipe and they do change the sound some also.
http://store.activeautowerke.com/col...e-bmw-f30-328i