it all depends on which engine you decide to run. different engines have different power characteristics, and different pipes exploit different power characteristics. An engine is basically just an air pump...and if you find a combination that will let it pump more air at the proper speeds and such, then you'll have a faster car

now that doesn't mean go and drill a huge hole in your carburetor...more air is better, yes, but the engine's basic design has to be able to USE the air that it gets in the most efficient way possible.

Also, when you get to upgrading engines (i'm definitely not trying to scare you into NOT buying a new engine, but this is something you'll have to remember) your parts breakages will increase, because your car will most likely be going faster when it hits something, and also if you go crazy with your engine upgrade, you'll start tearing up drivetrain parts left and right.