I think he lacks power for such a big guy. That should be his best weapon really. Browne backpeddled on a broken knee and had no defense to get out of the way. He went backwards and couldn't get his feet under himself. Bigfoot landed a shot that was wide open. Almost a free shot really. And Browne didn't really go out. He peppered Fedor and bloodied him, but didn't put him out. He obviously has power, but not as much as you would expect.
His lack of power comes from a pretty bad striking technique. On the ground he doesn't pick his shots, but rains down untechnical donkey kong punches. He sat on Emelianenko for the most of two rounds in full mount, but couldn't push the off-switch.
40lbs lighter Dos Santos generates more power with his punches due to his much superior striking technique.
I agree, Silva is a gatekeeper. I think all three of your suggested fights would be quite tough for him.