I second that as well.... I have a buddy with your exact setup minus brakes, diff lockdown kit and a couple other things but is on RB's AR downpipes, and cut the secondaries out and placed the "quiet vibrant resonators" (they have a super quiet, quiet ect..) and straight pipes all the way back with muffler delete...He has a very very slight rasp at 2200-2400rpm's but nothing like I just heard. I really think it makes a difference where they are placed for sound quality and where the secondaries go is kinda a perfect place.
I was an idiot and did the Akropovic setup downpipes back which was a WAIST of money IMHO.. Dont get me wrong, it does sound good but not for the money I paid. I can do exactly what you did or my buddy actually and get similar results without the money I spent :-/
Oh well lesson learned
A/W E92 335i M-sport with JB4 G5 ISO/ E40 WEDGE PERFORMANCE BACKEND FLASH, Alpina TCU flash for AT/Switch-tronic +AR Downpipes, Stett gen 1 intake, Wagner-Tuning FMIC+RB Turbo setup/ RB PCV Valve+ Akro exhaust+60% E85/93 mix,+ HRE P40S wheels, KW coilovers, M3 sways,+BMW performance CF Mirrors, BMW Performance CF deck lid spoiler,+ CF interior trim, p3 vent boost gauge,+ LUX angel eyes (11.73@119mph on street tires) NO METH!