[nyrfan]

1- An exhaust will give you NO HP gain.
2- An exhaust is just for sound
3- If you're just looking for improved sound the cheapest way is a muffler delete. Personally I do not like the sound of a muffler delete and went with a Corsa catback BUT I would not go the exhaust route again in the future and least not a catback and not to start, see 4
4- Go with LT/Cats. This will give you the best balance of sound and power.
5- If after the LT's you still want louder save some money and do an alxeback as the is really NO benefit to a cat back.
6- If you're look for just sound (no power) go with a Borla or Solo axleback.
7- If you change out just the exhaust and do not eliminate AFM, AFM does become much louder.