I did a little test on a 3700. Added side port (like a 4000 muffler) to the muffler case and drilled the diffuser holes out 1/16" dia larger. That seems to be the common mod most people on here do. The stock exit holes were left as is.
Same bar and chain, same saw, same wood, stock against modded muffler. The vid showed about 1/10th of a second diff. One could assume it made no noticeable difference to cutting speed at all. The noise definitely increased.
I've come across three different hole design diffusers on those models. If I recall, the hole area was different on the three.
Every saw is different and some saws benefit more than others. My 026 and 036 had very small exit holes and they definitely benefitted. The 3400 - 4000 series, I'm not so sure.
I'm am sure that quite a few people would disagree.
Whatever the outcome, it's always fun to fiddle around with them looking for a little more power.