[Menace]

The 2005+ GT's have a very good stock exhaust system. The pipe from the header down to the muffler is 2.5" in diameter leaving plenty of room for exhaust to flow. An axle back exhaust is just swapping out the stock muffler - which isn't restrictive as it is - with another muffler from the aftermarket. Since there is barely any restriction with the stock system, you won't see any, if at all, gains from swapping mufflers. All you will get is a different exhaust tone.

Now, if you change the pipes (The section from the headers to before the axle back) you might get some gains, but only if you change out the more restrictive catalaytic converters. You can either go with an Off-Road, or basically the deletion of the cats, or you can go with a high flow cat system. Though do note that removing the cats out of the picture is a no-no for emissions, unless you live in one of these states where that isn't a concern... but it's just a matter of putting in your old H-pipe once that time comes around.

Finally, there are headers. Shorty headers, which is what we have stock... can be replaced with aftermarket ones, but they don't have that great of gains over the stock. Long tube headers, which require a little more work, but have much better gains are another option you can go.

Hope this helps.