OP,
Hands down LT's (Long Tubes) are going to be the best for an overall increase in sound. We've run
BBK's Chrome Long Tube's on our
2011 V6 Project Car along with a
BBK O/R (Off-Road) X pipe for a really aggressive sound.
I would personally recommend looking at a chamber axle-back if you're going to be doing one as the sound can be on the higher pitched metallic side. Going with a muffler delete can be a little on the deafening side!
Regarding the gains I've seen 3.7L's make 290ish/275tq with a CAI and Axle Back (The Axle-Back's are primarily for sound and you're not going to see much of a gain with them as they're situated at the rear of the car and are essentially just a muffler swap) but after the LT's and X pipe you would see in the neighborhood of 310-315/270tq which is pretty quick if you're comparing this to older GT's!!!
***It's important to note that if you're going with Long Tube's they need to be attached to a
Shorty X-Pipe in order to fit correctly!***
If you're going to be shooting for Long Tube's and an X pipe it's great idea to pick up a
Bama X4/SF4 Tuner so your ECM can accommodate the additional exhaust flow. With any newer model Mustang it's the first modification I recommend doing as these new cars are just too smart for their own good! Installing anything aftermarket on them will make them act up for lack of a better explanation.
I hope this helps! Let me know if you're to have any questions at all and I'm here to help!
-Alex