Nope, it adds power, because of less backpressure.... The only thing your cat is supposed to do, is get really hot to burn up little particles of whatever California doesn't want in their air.... At least that's the best way I know how to explain it... Oh, and it's also a tool for automakers to make more craploads of money off of us, since they're expensive as hell to replace, once they go bad and make your car stink and lose power even more than usual....
In short, hack it off... If cops around your area are the type to get under your truck to verify that you have a catalytic converter, just go to a cheapie parts store and get a short Cherry Bomb muffler... Paint it black, with hi-temp paint, and BAM! Your new "catalytic converter".... Helps keep your truck mellow sounding, and fools the average po-po.....