I had the same concerns. My thought for going with Eisenmann Sport was if I felt it was not loud enough, I can always get an X-pipe. (Vs having to sell it and get the race). So far I'm pretty happy with it but I'd be lying if I said I havn't thought about making it louder at times.
Dev's right though. The best is to listen to it in person (if you can). If I get an X-pipe, I definitely want to check it out in person before I commit. Good luck.
09 E90 M3 DCT SSII Carbon Blk, Eisenmann Sport, RAC 19"RG63 on 255/275 UltracS, Dark Sidemkrs/Reflectrs/Grills, BMC Filter, LUX H8 V3 LEDs, V1 H/W, Ultimate Pedals & 30% Tint