Interesting points. Nice picture, too!
I do query whether the dear old Dak was getting any downwash effect at 95' above the sea (as I recall, the wingspan), so I am surprised at the quoted 1 x wingspan being the height at which downwash becomes useful.
Also, so far as the drag reduction is concerned, I think (from memory, but it's late) that the effect of the downwash reduction at low level is to rotate the effective lift vector forward, thereby reducing the component pulling rearward (i.e. drag) (I think it also fractionally increases lift).
This is different from the drag reduction gained from reducing wing tip vortex by flying low.
er, I think...