I'm hardly the chainsaw expert, but I can tell you where I would start
1. Get a tach. Not only does it take the guesswork out of tuning, but it will show you what works., and how well.
2. Once you have your tach, the first thing I would suggest is to pull the spark arrestor screen and see how many revs you pick up. You may be surprised: that screen is a substantial flow restriction all by itself. In fact I suggest richening the mixture and bringing the revs down below max BEFORE pulling the screen. That gives you room to play with before you over-rev the engine.
Also, I'm not saying you should remove the screen permanently. Whether to do that is a matter of what's legal and how brave you feel, fire-prevention-wise. The purpose in removing the screen for testing is to get a baseline and establish some "feel" for what the saw responds to when you open up the muffler.
SO once you've see how much gan is to be had by pulling the screen, you can put it back in and see what to do about getting that gain back by modifying the muffler in other ways. I'll let the real experts chime in on the best ways to do that. Just keep in mind that more is not always better: cut too much and you can actually lose power.