While I agree that done properly by someone with the necessary and skills for tuning these small two stroke engines, there will be an increase in performance. It's the ones that say things like "Opened up the exhaust on my saw, now it goes like stink" etc, that's what I find slightly bemusing. Yes it's louder, and sounds like it's faster and more powerful, but is it?
Also, going back to my days racing Motocross bikes, it was always the case that anything highly tuned, built for power and speed will need constant rebuilding to keep it that way. I guess these days technology and materials have changed so maybe they will withstand being pushed a little better. But I still believe it's a case of, increase performance, decrease lifespan.
After all the machines should be optimised from the factory for max power + Longevity + Reliability, once you start changing things you can ad those to a sliding scale, something has to give.