u mention keeping ins costs down.. the ins co goes by vin # so they know what u have base or sti.. u can add whatever parts u want to your car.. u have to have a special rider for some cars and the added parts to be covered in an accident or theft.. the manufacturers warranty is a different issue..they can void out the warranty as they see fit..shouldnt affect the warranty to upgrade the stock parts with another trim line of the same brand of car.. people add spoilers to cars that dont have them and that doesnt change the ins or the warranty.. the manufacturer and the ins co can let u know what increases the premiums and what voids the warranty..