No experience with light loads, but others have, and can speak better to your points.
FWIW:
I've only towed heavy (14,300 lbs & 14,600 lbs) with a large frontal area (wide body 5th wheel travel trailer). Tow-Haul mode works better for me at that weight. Especially in the mountains. Torque converter locks up earlier to control transmission temps.
The transmission programming in L mode could be better. under heavy load it wants to drop down to L2 at over 5K rpms, even under partial throttle. So I end up just slowing down, to keep it between 3,500 rpm-4,200 rpm. It can still out pull a loaded big rig up a 6% plus grade (not meant as a brag, it just does). I pass them going up, they pass me going down. Rinse & repeat... lol
On the flat steady state cruising I can't tell any difference.
Fuel mileage in tow-haul mode has been pretty reasonable. 7.5 mpg to 8.5 mpg. That is only a couple of miles per gallon off a stock '07 Dodge Ram with the 5.9L Cummins. However, the Dodge did a much better job of maintaining freeway speeds through the mountains, both up and down.
Happy Trails!