Brock would destroy him. He has better wrestling, more strength, more weight, and heavier hands. This wouldnt even be close...
In a pure grappling contest? Probably Roy.
In a pure striking contest? Probably Roy.
In MMA? Brock.
This makes a lot of sense, it's tough to tell though since we're all talking about a guy who's in unknown condition.
IMO Big Country takes the fight against Mir 8/10 times, he's already dominated him in a jitz match and he's improved insane amounts since then.