It's so funny that you just asked that. I was about to start a post asking the exact same thing. My point is, in contrast, if you are adding nothing more than more piping and some tips, why not just remove the rear section entirely? no tips could be even cleaner than the aftermarket options (if you're into that look). Would it be any louder than a muffler delete?
Jet Black 2012 BMW 135i E88 / M Sport Package / DCT / BMW PPK Stage 1 / Scopione Front Splitters and Rear Diffuser