I bought an MS201T in 2013 and have NOT been happy with it. Took awhile but I figured out how to fix it reading posts on Arborist forum. I followed the posts and it's a little hotrod now. Basically did 4 things.
Anyway it's a LOT snappier now and pulls HARD with the bar buried. I am impressed. It kinda has some 'run-on' when I let the throttle off. I can't get this to go away, it's like a mid-range leanness, not affected by the high or low screws.
If I read you right, taking your finger off the throttle initially has little effect on rpm's? I do not believe this is a "mixture screw" problem. This is more of a response problem to your throttle. There is something going on in the throttle to carburetor feed. If you think it's lean, check to see that your needle valves are clean. The slightest spec of dirt can create an ill. I am not sure what your attempts with the diaphragm do but bend it back and see if the run-on goes away.
