A turbo back exhaust system would cause a CEL right? Know if it does, how do you guys overcome that problem? Wouldn't your warrenty be voided? Or do you guys just install an after market computer in your cars?

Sorry guys, I know im asking alot of questions. The thing is im hopefully going to purchase an STi soon, and i want to do a few modifications to it but still keep my warrenty. Maybe I should just go with a good cat-back.

One more question. I just want to have an air filter, BOV, cat-back and a turbo timer. Would that be a problem with warrenty? Thanks for your help guys.

None of the mods you mention directly affect your warranty, but only the exhaust increases performance at all, and the turbo timer is entirely unnecessary. Technically, any modification other than a catback can be grounds for rescinding the warranty, provided the mod directly affects the malfunctioning part/system. Changing your air filter won't technically void your warranty due to a small clause in the Magnussen/Moss Warranty Act, but the STi filter is damn good. I wouldn't change it.