Same as I posted on the
www.m5board.com forum:
I did it and was rather disappointed by the results. My frame of reference was my M6 vert. When I did the resonator/secondary cat delete with an x-pipe, I got the F1 howl I was looking to hear, and being a vert vs. a well insulated SAV, it was significant. Now I may have a little more burble in M mode, no additional drone, maybe 20% louder at full throttle, minimally more snap/crackle/pop and no SOTP dyno improvement. Looking at the resonators, they are a straight pass-through design with no restriction that I assume is to protect the cabin from any drone, and I also assume the stock exhaust really acts as a x-pipe, too, with it's design. I was hopeful it would give me more of a V8 growl (like the AMGs get with similar deletes), but it seems the small displacement, TT's and the huge oil drum that is the stock muffler won't have it. I'm not looking for an open exhaust sound, just adding a more muscular sound.
I didn't take any before/after since I don't think it made much of a difference.
Bottom line - the $200 would be better spent towards some other mod. I may try the Megan Racing muffler as another cost-effective way to increase the sound without spending thousands on the big name with no performance gain exhausts.