A 10% power gain with a full system is nothing to sneeze at, and the better looks and sound is nice too IMO... 
Maybe 10% with reprogramming and other mods (like pipe). Slapping a replacement muffler on a bike, alone, will do pretty much nothing for performance. I suspect most of any power gain would have a lot more to do with the reprogramming and little to do with the muffler.
And really, after many hundreds of dollars of mods, is 10% even really noticeable to most riders? I am not sure it would be to me. I have nothing against performance, but 10% one way or the other doesn't open my wallet. Would much rather spend that money on convenience and safety mods, myself.
As for better looks- that is in the eye of the beholder. I still think the stock looks fine, and even better than some of the replacements I have seen. I think it is perfectly fine if a bike owner hates or dislikes the way the stock muffler works and replaces it with something more attractive to him. But I dare say that MOST people replace the muffler not because they think it looks bad, but because they think OTHER people might think it looks bad, or because it doesn't APPEAR to meet expectations for something that has perfectly fine performance.
And "better sound"- well, I addressed that already: To me, any increase of sound pressure in any frequency is not better, it is worse. Trading some frequencies for others- that is just "different" to me, not not "better".
