looking for more Horsepower

[Steve Tinker]

The biggest bang for your bucks is the Flat 6 underdrive pulley - check out their website at the top of the page.
Maybe + 5-6 HP gain, but the torque improvement at normal driving revs is an even bigger ++.
Leaves questionable exhaust systems for dead.....

[mikefocke]

Here

But the short answer is there is no quick cheap single thing that will significantly change the performance. There are lots of little things, each having some effect and cumulatively, 6-8 of them can start to really change things...including the thickness of your wallet.

But most butt-o-meters can't really tell the addition of 6-8 HP and some of the mods that claim that negatively effect driveability.

On the other hand, you can get well over 400 HP but it will cost you a couple of times what you spent on the car.