There are issues with GM engines and variable displacement. Basically the whole idea of cutting off 4 cylinders by removing pressure to the lifters dramatically shortens their life on the eariler engines. There are revised lifters and a reflash on some models. Also on a warranty repair any tech with half a brain just orders everything. You only get paid to fix the car the first time and GM foots the bill for the parts. So you replace everything to cover your ass so you don’t have to do it a second time for free.
IOM has your car ever thrown a p0300 code that is intermitent?
Also any GM dealer should be able to do your warranty work. I get stuck with pontiac and saturn stuff all the time.