Forget the de-cat, unless you running a turbocharged engine and are squeezing every last bit of power out the engine you can, I would perform a mufflerectomy - remove the centre resonator with a straight pipe and fit a good high flow back box, it would be quite loud with the stock backbox, but if you fit a decent back box you can have anything from a slight purr to full blown chavvy growl, depends what box you put on it, but either way, ditch the stock silencers. Still wont gain much power though.
