If they want to be picky they have an argument for voiding it...I'm assuming "modified headers" means it's an aftermarket exhaust as the GS did not have headers from the factory. If you had a tune done as well, that just ads fuel to their fire. I've heard from guys that have warranty voided for just installing a CAI. Easy answer is to ask your dealer's service manager.... and if he say's you're good with the warranty, get it in writing.