to add to what's already been said, you can use your stock gsr valvetrain with itr/ctr cams if you stick to the stock rev limit. any higher and you'll need at least itr intake springs.

any aftermarket cams whether stage 1 or stage 10 NEEDS new valvetrain imo. the extra $300-400 you save from not buying them will be cheap compared to having to do a rebuild when your pistons say hello to your valves.