[0] https://en.wikipedia.org/wiki/Issus_(genus)#Gear_mechanism
[1] "Interacting Gears Synchronize Propulsive Leg Movements in a Jumping Insect," by M. Burrows et al Science, 2013.
I don't know about that.
This platform can be fairly cheap in mass production. The legs are really cheap. No gears. And just two motors per leg. No extra force sensors, just the control electronics.
But it's not until people have played with the use-cases, etc, that one really needs to think about efficiency in terms of battery life.
That might help with one particular gait, but I think it wouldn't give as much benefit for a system as flexible as this, which can run, hop, trot, and more.
And the spring itself would need to be modeled, and would make the position and force measurements less sensitive. That's why I don't think it would be a net win.
Either gearwork or biologically-inspired designs, replacing some flexible electric rules and brawn with structures that have rules and brawn "baked in".
Sort of like how your knees aren't "soft-coded" for a particular set of angles, it's an emergent property of a system shaped by a particular use.