You could also get an improvement in performance if you open up the stock muffler all the way by removing the baffle completely.
You have to watch your carburation once you open it up, or if you put on and aftermarket can like Jeff's.
Finally, it's not so much the muffler, it's optimizing the distance from the cylinder port to the can...longer is good for higher RPM and shorter is better for lower RPM. These engines are relatively slow turning.