M Performance exhaust sounds great, but save the $$ and get the ACM Style Exhaust Mod. It is the exact same style as the M Performance exhaust (as confirmed by a BMW engineer)
The only difference it the type of metal. The M-Perf is made of Inconel, which is marginally lighter than the stock exhaust material and resonates (vibrates) at a slightly different rate. I would save the $1,500-$2,000 and do the mod which will get you sounding 95% just like the M-Perf.
Unless side by side the difference is really inaudible, heck even side by side its rough. Plenty of threads on the topic or you can call ACM and get more details.