GSP should be slower than he was in the 1st 2/3 of his career, especially if he bulked up. If he can get to the quickness he had in his last fights after knee surgery I think we have the most compelling match-up. A bit slower and he gets KO'd, no question. A bit faster and he just sits on Bisping and possibly rips his arm off.
That's why I think this fight comes down to how sharp and fast GSP will be in the Octagon. Bisping, at his best or worst, would beat a slower GSP. Bisping at his best would get mowed down by a prime GSP.
My best logical guess is that GSP will come in too heavy, too slow, and get rocked within 2 rounds. My next best guess is that GSP comes back a bit better than he was in the Hendricks fight and sits on Bisping for a win (or maybe gets KO'd). The least likely situation is that he comes back in prime shape, in which case Bisping would be wise not to show up.