The difference between the wheel size of bikes commonly ridden is just tinkering at the edges. I don't see people riding ordinaries because of their more efficiently sized wheels.
That's actually a very good point! You can't divorce just one part from the rest of the bike when you're discussing efficiency. Everything works together and, what's good in one respect, can be a serious drawback in another.
You also really to define what you are trying to accomplish before you can discuss efficiency. Bike Friday, for example, has carved out a nitch market for bikes with 20" wheels mostly because they are efficient to pack and travel with.