Nash dunking to win a game isn't that inconceivable. Nash can dunk, or at least could at one point in his career.
I know because I read an article on him during which he admitted players bet him he couldn't dunk in practice, to which he immediately dribbled to the hoop and stuffed a dunk, which he admitted didn't get much over the rim, but the players nontheless admitted he could dunk.
I suppose if the lane was completely clear, but I'm guessing he'd still lay it up.
ON MY OWN WEIRD WIS GAME NOTE:
I once saw a game (several years ago) in which Muggsy Bouges somehow blocked a shot by Hakeem Olajuwon and wondered how it happened. I mean, did Hakeem fall and try to shoot from his knees? Muggsy had some ups, but that's kind of far fetched.