We can only imagine: what if Obama-Biden had pulled a 4-game sweep in the debates?

They did not like Romney much and hemmed and hawed over the nominee. I think it was a case of simply falling in line after the convention. The debate was just an excuse for them to do so. They would have anyway because they can't help it. They hate the president and would have supported anyone who was not him.

but i think the house was pretty well insulated by the redistricting

It might even have resulted in a worse outcome for the Dems. I think that Obama was kind of like the brilliant student who was coasting along, and the first debate was a wake-up call to step up his game, which he did handily.

1. It helped overcome any tendency to complacency among Democrats, which was a real danger at that point in time.

2. It meant that the superPACs, which had just about given up Romney and were about to divert more money to downticket races, kept supporting Romney even when his poll bump subsided.

The rest is history.

Even if I accept your premise, in terms of states in the presidential race, what difference would it really make if the landslide had been any bigger than it already is?

In terms of downticket, it's possible there are new Democratic House and Senate members today as a result of what happened.