First, the Spoon axle-back is LOUD. You probably wont see much gain if any just by bolting on the muffler. As for the JDM header, it has a 2.5" collector, which means you will have to get a bigger cat or testpipe for it to fit onto your exhaust. If you are going to get a new muffler and header anyway, get a 2.5" exhaust or b-pipe to maximize your power gains.