Yeah, any Republican (or Democratic) nominee is pretty much guaranteed 40-44% of the votes just based on party affiliation (assuming there's no 3rd party candidate). It's about getting those extra 4-9 percentage points.
I honestly give Jeb a decent chance to get those few points. The Republican party is so extreme that when a moderate pops up, people will be drawn to them. Jeb seems like an old-school Republican which a lot of people think is ideal because they're "fiscally responsible" while not being fanatic about social issues.
The far right will bitch and moan about Jeb's liberal leanings but they'll fall in line.
Plus, there may be Democratic fatigue. Democrats haven't won the White House three elections in a row in several decades.
I'm too lazy to look it up but I'm pretty sure that even Republican voters showed a slight majority in favor of the Dream Act or some sort of legalization path for undocumented children.
Elected Republicans are another matter. There's only a handful that entertain that idea. The rest firmly believe in 15th century-style mass expulsion, or in making their lives so unbearable here that they have no choice but to leave on their own.