Anybody in his right mind would say keep your expectations low. He's old, he's been out of the game too long, hasn't had much practice this year, etc.
I say he will play great until injury does him in. Without a crystal ball, I'm at a loss to say when that will be. The Broncos are giving up the most rushing yards of any team in the league, so your estimate for Priest's numbers may even be a bit conservative. Play him. Nobody can fault you for starting him vs. the worst rush defense. Any great back is going to burn a defense if he runs free, and you have to believe that sooner or later against Denver, there won't be anything between Priest and the endzone but turf.