I've quite appreciated the quality of the discussion on this topic. Your question about the sweet spot where the transmission might be more efficient has me wondering too. The way I see it, the hydraulic part of the transmission has three modes. One where it turns the same direction as the mechanical which speeds things. The second where it just holds a gear straight (resulting in direct drive), and finally where it goes in reverse to slow down the mechanical side.

In all three modes, the hydraulics require enough pressure built up to prevent the planetary from free-wheeling and stalling the output shaft. I'm not up on things enough to know how much power just maintaining that pressure requires. I tend to visualize that the third mode would be most efficient as the hydraulics are allowing the gears to rotate the direction they will naturally want to go. Ie, the slower you go, the more efficient the transmission will become. Again, I don't know that. It's just intuition.