If the panels are 20% more efficient, wouldn't you need to install 20% less of them?

As an aside to the topic, that's not quite how the maths would work. If the panels were 100% more efficient, you wouldn't need 100% less of them

If the panels were 20% more efficient, you'd want 1 -1/1.20 = 16.7% less of them.

Yes, but that only becomes a win if the new panels cost less than 20% more than existing ones.

If they cost less than 20% more _total_, including installation costs, which is interesting because it means the higher the installation costs in a particular area the more likely it is to be worth the added cost of the more efficient panels (which presumably aren't more difficult to install than traditional panels).

Efficiency could lower labor costs potentially. Physically less panels to set up.

Bingo. Same exact cost of labor, 20% more power.