Anything more than one fight and prolonged success is a longshot, definitely. I have no real idea how good he is, but he's a pretty intelligent dude and I doubt he'd even entertain the idea if he didn't think he was at least competitive.
All I'm really saying is that I'd love to see him give it a shot if he thinks he's capable. There is no question he could cash in because he's a natural heel, an excellent talker, he's well known, and as a pro wrestler he automatically offends the purists in the audience. That all spells money in the UFC. He'd be a guaranteed draw, and win or lose he could return to wrestling and incorporate the whole experience into his gimmick if he wanted to.
On top of all that, since he seems so uninspired in WWE lately, something like this could give his career the shot in the arm it needs.