Don't throw away your money!
04' GTO has all the factory installed, best and most efficient airintake/exhaust system, period. You go ahead and put some KN airintake and you will have your LS1 gulping hot less dense air. Your parameteters will go bezerk and probably end up contaminating your system with oil. Your LS1 pistons will take in air by negative pressure and, as it is, it has all the air availbale it can handle. You don't need anymore because the GTO takes air in through the air dam, plenty of cold , dense air and routs it through proper diameter ducting into the airbox with a nice filter and then into your manifold where intake diameter gets much smaller so it does not matter what else you do outside. All sensors and software is designed for specific dimmensions of your system. The GTo electronically adjusts itself for the proper fuel/air mixture (See your silver GTO catalog that explains all of that). The KN airintake system, as well as all others, what it does for you is to spoil the whole design to allow hot, under the hood, undesirable useless air into your car's engine probably shortening its life.
The GTO exhaust system has been carefully designed to maximaize air flow from your engine. GTO engineers went at great lenght to design this system. Please, don't spoil it with some off the shelf magic pill type of solution. Don't fall for these aftermarket useless ideas.
Only way to actually increase significantly your vehicle's hp is to modify the engine which would take some finely tuned engineering to design, build and mantain. Unless your last name is Ford, Holden or GM don't get into it. Keep your GTO as it was designed for the job it was designed and take your money to enjoy a meal out with your gal. It is a powerhouse and a fantastic car as it is. It is a beautiful piece of engineering. If you still want more power get an 05' GTO.
Have fun driving it! I do.