From my understanding, 99-04 GT's flow damn good with a full 3 inch exhaust without any cats. Would it even be necessary to get LT headers? I have heard that there really isn't to much of a gain, because the exhaust is flowing so good in the first place. For the money, is it even worth it?
LT's WILL give you more power across the power band. peak #'s arent much different but getting 15-25 across the board....yeah, theres a bit of a gain.
Kilgore makes a good point, they do cost $$$ plus a lot for install. but if you're in the modding game, you gotta pay to play. I'll ante up the $700 for headers and install them myself.
look up Niickthegenius and click the link in his signature: "Thread of the Century". good read on LT's and the misconception that they dont produce power on 4.6s.
unless you're making a lot of power (ie, Forced Induction) a 3" exhaust system will cost you low end torque. the stock manifolds flow pretty good compared to aftermarket shorties, but LT's will give you more power than either.
fact is Longtubes are a great bolt-on and will give you some good gains throughout the powerband. peak #'s may not be much more than stock manifolds but you're still getting 15-25 throughout. remember, your car isnt always operating at its peak power #'s.
if you dont wanna spend that much $$$ (partshopper is selling Mac LT's for $299 right now, not a bad price at all) then dont even bother with shorties. put your money somewhere else.
I immediately noticed better/smoother power,especially in the midrange after the LT install on my car. I also felt it justified for the nitrous use(200 shot)