cold air intake: Get a wiz or make your own
k&n filter: Just a drop-in is cheaper then a CAI, but not as good performance gain
new exaust: I'm assuming mufflers, don't worry, the stockers aren't very restrictive
new headers: Headers will give more performance, but do your homework on them
platinum spark plugs: NEVER in these cars! The factory plugs are Iridium. Either stick with the Iridium, or put in a cheaper Autolite 605 copper plug.

Quote Originally Posted by sgdude View Post
my goal is 300HP (a 60HP gain), will the mods above get close to that?
The factory HP rating is flywheel HP. The WHP of a stock GTP is close to 200, so if you want 300 HP to the wheels, you'll have to get the motor up toward the 400HP range.

To answer your question: No, the above mods will not net you even 300 Crank HP. Add a pulley drop and a good tune to the list, and you'll be closer to where you want to be.