this is a hypothetical question wrapped inside of a "what if" wrapped inside of a "who knows?"
But because it's a rainy Saturday and I'm tired of talking about Seymour, I'll take a crack at it.
Of the five games the Pats lost last year, three were by 25 (first fins), 20 (Chargers) and 23 (Steelers) points. So I'm going to say that even if the Defense had played "really well" (once again, hypothetical, "what if" and "who knows" what that means) and held the opposition to 17 fewer points we still would have lost those games.
The other two losses were by three points each (Second Jets and Steelers). So, if the defense had managed to give up three or better fewer points in each of those games we would have had a chance at winning each of them.
Playing this like a probability game, then, I'd say it was 50-50 that we would have won one of those two closer games (once again under a hypothetical, "what if," "who knows" scenario) if the defense had played "really well," whatever that means. However, I note that I regard that analysis as practically worthless.
Otherwise, I think this thread suggests that it's really, really, really time for the season to begin...