It's too much flow for a stock valvetrain to do anything with. It will kill his flow velocity and gut his low/midrange torque in exchange for small HP gains way up high in the rpm range, he'll be slower overall. The GSR IM flows just fine for a bolt on's B18C1. It flows enough to support over 200whp, it's when you get advanced far enough in your NA build (with upgraded cams and compression) that you want to localize exactly where in the rpm range your powerband is located and increase flow in that area, to have peak volumetric efficiency occur. That's when you shop around for the right size IM, to complement that powerband.
I don't consider an IM a "bolt on" mod, there's alot more to installing an IM than just bolting it on like an exhaust, header or intake. An IM is a more advanced mod than most people give it credit for, I think many people get one just because it's relativley inexpensive, it looks good, and they believe the advertiser's BS HP claims.