Unfortunetely our cars do not respond as well to bolt ons as other cars do. BUT, bolt ons can be the supporting cast for bigger and better things such as high compression pistons as Jim said above. Intake/header/exhaust may give you 10 hp with a stock 2gnt, but you raise the compression and that number may jump to 15, or 20. Get some headwork done and the header may give you ten by itself... this is why its hard to give exact numbers. Too many variables.

Check out the main page, under sections you will see stages. This is a quick rundown of some numbers you can expect from the most common mods. There is a TON of information on this board, probably an answer to any question you could think of. You just have to look a little bit.