Enzymes convert starch to sugar. It makes sense to me that an increase in enzyme activity would increase the efficiency of converting starch to sugar.
Not sure that's the way it works. The increased activity that happens at lower mash temp is more about converting more of the more complex (unfermentable) sugars into simpler (fermentable) sugars thus a more fermentable wort and lighter body. But, regardless of whether they are complex or simple sugars, they all count toward gravity and hence efficiency. That is, 80% efficiency means you got 80% of the potential out of your grain. It says nothing about how much of that 80% is fermentable and how much is not.