I have a 20" on my 261 and haven't had any problems cutting southern hardwoods. If I have to cut the really big stuff I pull out the MS660 and after cutting with it I'm glad to go back to the 261! I mix Red Armor at 40:1 as recommended by
@huskihl and like I said no problems at all.
You’re right, the 261 has no problem with a 20”, BUT you do have to make a choice on the setups you run when you get where you are going. Since I have a CAD, I have multiple saw cc classes depending on what the job size is. Sometimes the 261 can do it all but I don’t usually swap bar and chain setup unless I have to. So for me, when dogs deep in a large
buck with my 261, I’m WOT for extended times and if your not thinking about what you’re running you’ll end up with a nice top-end, and maybe more, repair job on your hands,
While on the topic of setups, you can, and likely already do, end up having a bunch of bars lengths, chain sizes, cutter types along with the rims for each that will just be hard to manage unless you want your shop to look like Buckin’s. I speak from experience and I too like BBR’s saw shop but it’s not easy in the wallet.
I’ve seen tree service “wannabes” show up and tackle storm work tree jobs with the one and only saw they own from a big box store and just bear down until something gives up, and that is usually the saw because the guy on the trigger doesn’t have a clue.
This could end up as another oil war, but I only run MOTUL 800 2T in my big saws, and usually Saber in the smaller stuff, including my supporting 2-stroke equipment. Regardless, everything gets 40:1. If your have to run the Stihl Ultra, make sure your pour. is heavy and not just use that little bottle and go.