you might want to check with the local Kia dealership and ask the Service Manager what you can have added that wont void the warranty.
EDIT: After talking with our service manager here at the store. He said you could add the catback exhuast and a cool air intake to your spectra.
As far as adding a turbo kit, Kia doenst make one if you added one and if it were to do irreversable damage to your engine the warratny would not cover it.
Best bet would be to get the Kia parts that they have and try them, may not add a lot of HP to it but the Spectra does have more HP than the Corolla
