Most people with test pipes will agree that there is a big difference in the butt dyno with 'em. I dont have one on my car yet but from what Ive heard many s2000 owners with test pipes (some who are supercharged) pass emissions without cats. Thats not an exageration. If your test pipe will be custom made ask them to make mounts to keep your cat heatshield in place so it looks like the cat is still there in order to fool the emissions people. Good luck.