It depends on dealerships. Some will stock aftermarket performance parts and some don't. Warranty is a touchy issue when it comes to having aftermarket parts. If you lower your car and you blow a shock, it is highly unlikely that they will cover the blown shock....for example. But let say that you changed the muffler and your engine blows, they will have to proof somehow if changing the muffler had anything to do with your engine blowing up.