3" exhaust or 3" tips? I dont know what you've done to your car but 3" exhaust is overkill for most applications. I've heard you may end up losing power.
Mac makes both 3" exhaust and 2 1/2" with 3" tips.
I've got mac shorty header, 3in prochamber and 3in catback and very little mods. I lost absolutely no low end torque and gained top end power. I guess maybe the prochamber gives it enough back pressure to run well. I love the way it sounds, not like all the other mustang/flowmaster combos out there.