Anyone used plisson size 20?

Do you think I should try using it?

That depends. I had a plexi HMW 20 about 10-15 years ago. I used it a couple of times with creams, but it was too much of a good thing and I sold it to a collector for what I had in it.
I bought this new old stock 18 around the same time as the 20 because of the beautiful handle, and the old Plisson HMWs were characteristically scritchy, like the old Somerset made Simpsons. A characteristic I happen to like in a brush.

My 18 is scritchy the way the old Rooney Finests and Somerset Simpsons used to be. I use it to face lather off a puck and it is large but a delightful brush. If you've lathered with anything approaching that size you'll have an idea what you're going to experience. It'll be a lather hog, but the old rule of thumb was larger lofts for creams, shorter for pucks. Try it with a cream. A brush like that won't depreciate in value because it has been used once or twice. It's a collectors piece. Beautiful handle.