He'd respond, no doubt, by saying that his cuts are offset by eliminating loopholes. But until he can say how much money a "20% across the board" cut would cost, and how much would be recaptured by closing the loopholes that he's yet to specify, and about which experts say that eliminating every deduction (and, presumably, the ability to hide money in Caymans) you can't make it balance, he's a liar. Period. You can say he "won" the debate -- which he did, on style points -- but he did it by lying and contradicting everything he's said until now.
Making it even more clear than it was before: we have no way of knowing who Mitt Romney is and what he believes. In fact, I think it's pretty clear: he believes only in winning at all costs. Lying and changing positions at will doesn't enter into it: if it helps him win, it's justified.
What would he do after he wins, if he does? No one knows, not even Mitt. To him, it's beside the point.