Efficiency in the case of rigid-wing hang gliders pretty much boils down to less drag and more lift. My gut says those are not just correlated parameters though, there's got to be more about it than just drag. The differences result in higher speeds for the wing at the same glide path (I think this compares to higher VMG) or insanely flatter glide paths (pointing higher) as compared to flexible wings, Part of the equation is definitely having supporting structure inside the wing. The leading edge ultimately "becomes the mast" as it were at a certain point. I seem to recall that having a separate upper and lower surface gets rid of lots of parasitic turbulence, but I don't recall the theory all that well, so I'm not sure.

The thing that gets me comparing wings to sails is that wings on gliders (and rigid-wing hang gliders) are not all that complicated but very efficient all the same. I don't see why sailboats should need so much more complexity to use a wing sail and be competitive, whereas all these gliders don't...