Here are the current results if you average together all of the polls for the general election match-ups:
Hillary beats Trump by 8.5 points
Hillary beats Cruz by 3.0 points
Kasich beats Hillary by 8.0 points
So if the GOP goes to a contested convention then it seems like they have no choice but to screw Trump and go with Kasich if they want to beat Hillary.
But if they give the nomination to Trump, and Trump loses to Hillary, will Trump supporters regret that they didn't support a candidate that had much better odds of beating her?
RealClearPolitics - 2016 Presidential Race