To the best of my knowledge, the warranty is only void if the parts that were altered directly cause the failure of an engine component. Deleting your mufflers isnt going to cause a valvetrain issue, so that would be covered. If you did cams and had a valve issue, it wouldnt be covered. Nothing againts your friend, but it varies from company to company, and to the best of my knowledge, GM's warranties are pretty good when it comes to mods. Search on the forums a little bit, you will find that a lot of people have had warranty work done even though they've modded.