Per the Magnusson/Moss act, GM has to prove that the headers caused the mechanical failure you're bringing the car in for. Like I repeatedly post, if you put them on and your power window motor fails, they have no grounds to deny you warranty coverage.
However, if you install those headers, the headers allow the engine to breath better, it breathing better means it'll rev up quicker, and if you let it get away from you, bounce it off the rev limiter a few times, bend pushrods and possibly other damage, then they do have legitimate feet to stand on and deny coverage and there is nothing JBA can post or say about it. That is unless JBA wants to foot the bill.
However, there are plenty of GM dealers that follow GM's letter of the law and if they see ANY (and I do mean ANY) non-OEM modification, they'll deny you any coverage. That's where you can say something and use that Mag/Moss act to argue.