Bentsen probably helped the ticket, and I agree that he would have done quite well against Bush 1 v. 1. That being said, Bentsen may have appeared more 'Presidential' than Dukakis to many voters and this apparent awkwardness may have blunted the positive effects of Bentsen in terms of affecting the overall outcome.
IDK, he was a great debater, but I'm not sure how much VP picks and VP debates matter? If we accept that Bentsen swayed a meaningful number of votes in 1988, that leads us to the conclusion that Biden definitely saved Obama in 2012 and Kaine probably sunk Clinton in 2016.