I mean that they lose efficiency with increased torque. So the power loss percentage will be highest in hill climbing situations, where you have the least to spare.

Of course actual power loss will always be higher at higher power levels because we're X% of a bigger number is bigger, but that's pretty obvious.

By contrast, derailleur systems are most efficient when larger sprockets are used, so there's a small (not enough to matter) improvement in low vs. high.

In any case, I don't think the differences are all that material in the scheme o things, unless you live on the side of a mountain. Efficiency is only one of a bunch of factors to consider if trying to decide between IGH and derailleur. I use both, and prefer IGH for utility riding, and derailleurs for sport. Because I use IGH mainly in the flats, I would ratehr see 3-5 speeds which are mechanically simpler but with closer spacing, since I don't need the range for my applications. (I currently own a vintage SA 3s ultraclose spaced freewheel hub, which is perfect for flattish terrain, though I'd want something a bit wider (what SA used to call medium spacing BITD) for general riding in lightly rolling terrain.