Probably this:
Hillary Clinton 323
Rand Paul 215
Paul ceiling:
Hillary Clinton 270
Rand Paul 268
Hillary ceiling:
Hillary Clinton 358
Rand Paul 180
I think Paul would have a fairly high ceiling. There are people who would vote for him, but not for a Jeb Bush, or (at this point) a Chris Christie.
One problem is that he's not a completely known quantity, nor has he been a national figure long. He could screw up badly in a national campaign and get clobbered.
But he could also run a better campaign in an environment that's better for Republicans.