I don't think I have put 20 tons on them yet, but I have put well over 10 tons, likely closer to 15 on them. I estimate this because my air over hydraulic needs 175 psi to reach its max of 20 ton. I have my regulator set to 160 psi so I know when I am approaching the limit. Last few pumps on the jack I do by hand. I have had to do that a few times. Don't think I could ever say that I have seen 20 ton because that would mean the press stopped. I haven't experienced that yet either.
My bed rails are spaced at roughly 4.15", the aluminum you see in the photo is 4" in diameter. I haven't seen anything that showed these plates flex or deflect under the pressures I have applied. YMMV