I am an SLP dealer and me and the guys there have been round and round with warranty..... The packages are awesome and the price isn't that bad, we-as a Pontiac dealer- are warrantying the cars that we install parts on. No problems yet, but I'm sure that with SLP's quality we won't have any issues. Just an FYI, I've got everything on my car that is in there 421 hp package less the rocker arms and their tuning and my car runs a ton better!!!! I havn't had the opportunity to tune and thats because I'm going to do their head and cam package (thanks to tax return
) and then tune all together! Don't forget....with added power you must upgrade drivetrain components!!!!!! We are also a BMR dealer which is a leader in GTO drivetrain and chassis parts!!! Send me an email if you need anything and on prices!!!!!!
gtoparts@alanyoung.com 
