In terms of mobility, I really do like the McNabb comparison for Geno Smith. McNabb was a QB who had that ability to scramble, but was always kind of a bit clunky when he did it. Everybody recognized that McNabb would have to be a great pro QB with his arm, and not his legs, though there were still idiot football commentators that said, "I don't know if a running QB like McNabb can win a Super Bowl in the NFL."
It doesn't hurt that he had the arm talent but came from a wide spread system, and that he was drafted by Andy Reid.
Here's a comparison I like if Geno doesn't reach his full potential but also doesn't bomb out of the NFL: Matt Schaub. I assume not reaching his full potential would mean he never got control of his lower body as he drives throws, giving him only adequate arm strength. In the NFL, that will get you pretty far, but it will also not allow you to perceive what's around you in the pocket and step up when you need to. Schaub's problem is also that he just can't hit those timely completions the right way when he needs to make them. I can see Geno being that type of passer if things don't go as planned.