Contra to those who think that Supersprint is the only answer, there are other viable options for far less and just as effective. Honestly, Supersprint, while nice, is overprice nearly on the same threshold as Dinan. If I were looking for a non-custom exhaust, I'd be looking at Eisenmann, Magnaflow, Remus and other manufacturers. I have the Magnaflow 14815 which is not expensive, not expensive to have put on your vehicle by any competent shop (think $75 or less), and sounds decent without the deep, loud constant variation described by someone who sounds clueless. Is it the shizbang --- No but it does not make sense to spend $2000-3000 for a Supersprint which will yield you marginal gains just to say you bought it.
2011 BMW M3 ZCP in Melbourne Red, Apex wheels (ARC-8 and EC-7's) with Bridgestone RE-71R's.