I know I'm going to regret this, but...
Regardless of how much or whether lateral energy is returned or wasted, how can the measured deflection possibly affect that?
Take rider A on bikes X and Y. Rider A always has an identical pedaling style, and puts 5w laterally into the BB. Bike X deflects 4mm at the BB under rider A's 5w lateral. Bike Y is stiffer and deflects 2mm at the BB under rider A's 5w lateral. Bike Y deflects less, but still absorbs 5w laterally. In other words, the less deflection of the stiffer bike doesn't mean there's less wasted effort deflecting the bike, just more resistance to the same wasted effort. It would be like saying that a stiffer car suspension makes the road smoother.
Now rider A may prefer the feel of bike Y, of course. Bike Y may be sharper-handling. I don't see how bike Y can be more efficient though.
This of course also means that "planing" (which is the damned stupidest word for the suppose concept) is bunk, because deflecting MORE also doesn't mean anything better either, inherently.