When we engineers see "efficiency" we first think of nautical miles per pound of fuel.
This is seldom a concern with flaps down. Or with the military. under any condition.
The increased "efficiency" of a horizontal, flaps down might be considered a gift rather than something to be sought, unless there's something seriously wrong with the horizontal in the first place.
There;s no doubt a bunch of trade-offs in that area; adequate control for the worst flight condition, yet minimum interference at the best.
I see a lot of bottom views of the commercial jets going in and out of Burbank/Van Nuys, and there's two obvious but equally successfu contfigurations.. short tail moments, large horizontals and long tail moments, small horizontals...
And in the case of the MD-80s.. how does it fly at all? They took the wing too young from the mother, before it had time to grow up to be a real wing!