According to the November 2003 issue of Superbike magazine after testing three top-end slip on systems (Laser, Akrapovic, and Micron, and dyno-testing on a CBR600RR, they found that the average gain was roughly 3-4 peak power gain when mated with a properly-tuned PCIII. (Of course, the Akrapovic was slightly better in the dyno charts...

)
Or - roughly a 3 percent gain in power. Point being, today's bikes are so well-designed right from the factory that adding aftermarket parts and gaining some 15-20hp is just not possible anymore. Yes, you can realize some significant weight savings and get higher-quality materials and build quality, but the bottom line is that bikes today are being built to squeeze close to 100% of the juice they can muster without sacrificing too much long term durability. Stock parts are better than ever today. It's just that the industry is so competitive that people are willing to pay big bucks to extract every last possible pony out of their rides. 3-4 hp out of a slip on is actually not an insignificant gain from a muffler and a short section of pipe. But it sure ain't earth shattering either.