I've used my saws for 3 seasons of non stop full throttle commercial day in/day out use with the muffler bored out with absolutely no problems, in fact those saws are still running better than when they came out of the store. They've seen well over 2500hrs with nothing more than fresh air filters and spark plugs as needed - absolutely no problems so far and I'd say I've put those saws through much more than the "average" user might.
I have a hard time believing you don't ever pull out your little screwdriver and adjust your carbs and that you've never rebuilt your carbs? No rings?
Ozibatla is not "wrong", but it's not neccesarily totally correct either. Most saws with the new EPA-BS muffler end up getting tuned too rich if tuned by ear (a tach will show they're often "under" rpm, even out of the shop).
When you open up the muffler it does in fact allow more air to flow, but the carb doesn't necessarily self adjust to put out that much more fuel to fully compensate so most of the time you just end up in the optimal rpm range for the saw by accident with a small muffler mod. If you were already on the rpm WOT no load spec for the saw, then in all likelihood, the saw would end up too lean and over rpm'd. EPA makes the manufacturers put on little plastic limiter caps that limit the amount of fuel you can jam into the cylinder so you do have to be cognizant of how big you go.
Unless you really gape the exhaust outlet (hehe) it is a really minor change that needs to be made (or none at all) - even my brazil muffler on my 066 (two 1" round holes, gutted baffles) was maybe a 1/8 turn more than the single outlet muffler. The big bore/port/squish tighten plus muff gape-age resulted in almost a full turn further open to keep the rpms below 15000. My modded ms260 required a MS440 carb to fuel it - the little carb just could not do it even with drilled jets [HTML_REMOVED] bored venturi.
regardless, I think we would all agree: a sharp chain [HTML_REMOVED] any motor mod you can do (for a work saw, anyways)…