The stock exhaust on the GT is very good straight from the factory. Adding an axle-back or even a cat-back exhaust will NOT add more HP, it will increase the sound, which IMO is just fine as it comes. In order to achieve more HP in the exhaust system, you need to go to long tubes and probably shorty headers. A custom tune, CAI, and inceased air flow will do more than exhaust. Of course, a supercharger trumps all.
The mods game can quickly get out of hand, and you can spend a ton of money real fast. It's like getting "footitis" in the boating world. Mods may also void your factory warranty so some wait until after the warranty is gone before modding.
I intend no mods other maybe dropping it about 1" and maybe a new air intake down the road.