I have a MAC ProChamber with the cats, and Flowmaster original 40's welded in on the stock 2.25" pipes. It's not loud and it doesn't rumble at an annoying level (yet) but it does sound nice still, especially driving away from people. The reason the 05 GT sounds louder is cause it came with 2.5" piping already. Muscle Mustang's and Fast Fords are doing a 3 part exhaust section on it as we speak and they say why it sounds louder even at stock level, the piping makes a difference. I'm saving up some dough to be able to lose my exhaust and get a 2.5" original 40's Flowmasters again, some off road pipes for my prochamber, and some JBA shorties with a K&N intake...
Flowmaster might not make the "loudest" exhaust note, but it definitely makes the best and lowest tone in my opinion...there's nothing like a Flowmaster sound, I don't personally like the loud exhaust's, or the one's that pop too much, I love the low and full tone exhaust sounds, the one's that make your ears ring and fill up with sound, and make windows rattle