Matt Barrow writes:

How does the airframe "gain efficiency" at altitude, aside from generating
less drag?

Less drag is a big advantage. The higher an aircraft flies, the more easily
you can move it forward. Ultimately, you end up in outer space, with no air,
and then you don't need any energy at all to keep it moving forward (the
principle behind hypersonic aircraft). This is one reason why the SR-71 is
very efficient in cruise. It's also a reason why jets fly high even though
the engines produce less power at high altitudes--the power required to
maintain a given speed decreases faster than the power provided by the
engines, as altitude increases (within certain limits).