"On the other hand, greatest efficiency of a hub gear is usally at the unity ratio, with the gears idling -- and this is best as the most often used, level-ground gear, which is much closer to the top of the useful gear range than to the bottom."
"Best". I would say rather that it is a reasonable compromise to have the unity gear (probably the most efficient) to be the most used gear - if there is such a thing. I would also say that it is another reasonable compromise to have the most efficient gear be the lowest ratio - the one the rider uses when loaded/going up hill/against a wind, when the going is toughest - like the S-A 8.
And I hate to sound like a broken record, but everybody talks about efficiency with IGHs, but rarely with the various cog/chainwheel combinations of derailleur gearing. Yep, the efficiencies are different, and sometimes comparatively quite low with the relatively tiny cogs used to get useful ratios with small wheel (folding) bikes.