I too am a fan of Shane Carwin. I heard all the talk of him coming out of retirement but to date have not heard any more. I agree that under the right circumstances he could beat Cain Velasquez. But, I wouldn't bet the fight.
And the video you posted of him beating down Frank Mir is pure gold!
By the way that makes two very severe beat downs that Mir suffered at the hands of wrestlers. Brock Lesnar tuned him up as well. It seems that if a really good wrestler understands enough BJJ to stay out of trouble he can win.
Here's hoping Carwin comes back. Certainly one of the most exciting heavyweights to ever get in a cage. His career was too short.