The prochamber imo will make your exhaust a bit quieter. It has a deeper tone to it. I mean it will still be loud, but not as loud as a x pipe. The car should sound nice with the flowmaster's. Iv heard car's with american racing headers, mac prochamber, and mac catback and they are so sexy sounding. loud and aggressive.
I forgot which hpipe I had, but joe at mustang magic told me to do the prochamber, and I gained 13rwtq with just the swap, no tuning. before and after same day dyno.
I had borla stingers, off road chamber, and stock manifolds, and the car was retarted loud. If I did long tube's, forget it. Now I have mac long tube's, prochamber, and magnaflow catback. Nice and quiet cruising around, no drone, aggressive when you nail it, but it doesnt draw attention to cop's when driving around town.