I thin discussions like this are what MMA is all about: 'what if..?'
I would love to have seen Tyson in MMA. He is surely the hardest puncher of all time and yet he moved with the speed of a fighter 2 or 3 stone lighter. The quality of opposition in his early fights might have been questionable but the devastation he inflicted upon them was not.
I think it's pretty obvious that anyone in the UFC could kick all our asses but would any of them frighten us as much as Tyson?