I don't know about Romney's numbers. More relevant: McCain carried 45% of votes in 2008. Romney had to get to 50%+. He didn't do it and the "47% comment" hurt.
It's not so much that people dependent on government didn't vote for Romney (or Bush or McCain). It's that the comment made Romney look bad with swing voters.
That's the way it is with a lot of these "gaffes": the people the "gaffe" refers to may not vote for you in any case, but the comment hurts you with other voters who might have been yours to pick up.
But that's tough for that scenario to occur in '16, especially when it's not too difficult for mainstream media to stir the voters into a frenzy and pull the level for the Democratic candidate once again.