Seems counter-intuitive that a fuel issue would cause the saw to increase compression and then pull harder. If anything the compression should reduce over time and it should roll over easier.
Only exception I could think of would be a clogged muffler that would reduce the ability to breathe properly. That would build over time but I can't believe if it was clogged to that point it would even run.
My guess would be that you have binding in the starter mechanisim or possibly a bad bearing in the engine. You did however say the engine rolls over freely with the plug removed and that would rule out the bearing.
I have a 2-year old MS250 and that has serious compression. I have to step on the handle when I pull it but it starts easily. I am using the Stihl synthetic oil that should be giving me an edge.
Octane rating only applies to the fuel's resistance to pre-ignition (knocking). It has nothing to do with the energy content of the fuel and/or any cleaning ability of it.