Thanks, that tends to confirm my views of sticking with stock. I've always been a little dubious at posts from people saying they've removed the cat, added a KN filter, and that the bike is smoother / a little more powerful. I'll keep an open mind until the day I try it myself (if that ever happens), but unless one retunes the engine (or retunes itself!), I've always found it hard to believe how some guy in a shed could improve the engine performance from an engine that took many engineers to tune. The only doubt comes exactly from that - pollution limitations (and ECU ajustments). In theory, I guess the engine could run better without the cat and with more air, but indeed it makes sense that it would need more adaptations. I guess this is where the electronic gear comes in, that or the actual reprogramming.Adding high flow filters (more air) and removing the cat from the exhausts (exhaust gas exits more quickly) makes the engine run even leaner. This will cause the engine to produce less power, worse throttle response, possible detonation/ping, burnt exhaust valves. From factory, the engine's AFR's are at the absolute limit of the safe area, so adding intake/exhaust mods typically exceeds these.