What I've heard from alot of people is that the Bolt-ons dont give you alot of gain individualy but when you put them together, they add up and gives a noticeable gain.
Iam not alot into the performance mods, I've only got a K&N and a TL-Sway bar and thats all Iam gonna do but hey its your car and your money, do whatever you want to do.