I think Borla and Magnaflow also make an exhaust for the mp3, both catbacks
both very good exhausts, not might have to check with someone who has them?
If it were me i'd just go with a midpipe. Or just go with some larger 2.75" custom piping from header back. Thats what i plan on doing, cause i love how quiet the stock Racing Beat muffler is, but thats my personal preference.