I have a 5020 and the muffler is just a hollow can as it is. The only mod I did to it was drill a few holes to allow better breathing than just through the stock hole. I did tune it, and that made all the difference in the world! I run an 18" bar with a Stihl RC chain with no issues. Just gotta let the saw do the work for you.
Get you the correct carb adjustment tool and set the carb, they come like they are to meet epa, of course saws you get at the dealer has been properly setup.
Nothing really to mod on these mufflers. IMO it flows fine. You could open the outlet up some if you must but I dont see where any gain will be made on this open can muffler.
Tune the carb. Mine needed tuned out of the box. Wouldnt even stay running. Need a spine carb tool or redneck make one.