Take another look at what ACM is really doing: http://www.m3post.com/forums/showthread.php?t=609950
They're only bypassing the absorption chambers in the OEM exhaust, while keeping the Helmholtz resonators intact to maintain the M3's sound and minimize drone. If you unbolt the entire axle-back section and put up straight pipes instead, you will sacrifice the Helmholtz chambers. You'll likely have a drone monster on your hands at that point.
Even when Subw00er and Periokid did their mods they changed the plumbing post-Helmholtz.
I have the ACM style mod on my car and I love it. There's a reason that ACM, Dinan, and our other forum members saw good reason to keep Helmholtz chambers in place.
(Below images courtesy of Dinan.) Best drawings I've seen that show how much time and money BMW-M spent on designing our original exhaust to A) meet BMW expectations and B) meet U.S. federal requirements allowing them to sell these cars on our shores.
Dinan's explanation: http://www.dinancars.com/university