I don't think going from stock manifold to header on a GSR is a BIG difference, Whats it like 6hp or so... I would think the diff. from 4-2-1 to 4-1 could only be a couple hp anyways... If you have one or the other it def. wouldn't be worth the time or money to swap... I have always had 4-2-1s on my cars but have installed many a 4-1...