Lots of ifs. When he's been healthy he's been "decent" not elite. He's missed almost 40% of his games, and is coming off an ACL. Hard to know what he'll be like since he's missed most of the last 2 years. It should also be noted that he'd be moving from being a dome QB his entire career to the wild weather of Buffalo.
Certainly he'd be better than Manuel, who looks more and more like a wasted first round pick, but whether he'd be that much better than Orton or even Fitzpatrick is still up for debate. And THEN there is the near impossible task of putting together a reasonable trade scenario without a #1 pick and only six selections this year.
It's an interesting thought but has the same kind of likelihood as Harvin to the Pats.