I used 12" vibrant. didn't kill all the rasp but did remove alot of it and moved what was left to higher rpms. If I could have fit 18" in the mid pipe I would have used them. 18" probably would have removed all of it.
Recently had my car dynoed and I over heard the shop guys saying how good it sounded.
2012 Mustang Club of America Edition Silver Ingot V6 Premium, 6MT, BBK Shorty Headers, Mak Performance catless H-Pipe (not MAC), Magnaflow Street Catback, Airaid CAI, MPT 93R Tune, 80mm Throttle Body, Ported Intake Manifolds, SR Lowering Springs, SR Adjustable Panhard Bar, Lakewood 50/50 Shocks, Steeda Heavy Duty Strut Mounts.