No, voltage is easy, power is work.
Crystal headsets = high impedance, high sensitivity
Big, power-hungry magnetic coils = low sensitivity
Sure, engineers try design better, and will, but power isn't voltage.
Not sure I got your point, but looks like we're just using different terminologies rather than different logics ... Some words may have different meanings, depending on the way you look at it, hence I wrote the meaning ("measuring units", kind of) of anything in brackets.
Anyways, OP has to notice there are two kinds of headphones in that aspect -
the less efficient ones that are intentionally designed for use along with a headphone amplifier;
and the more efficient ones that will provide pretty much all they can do, directly out of a portable device.
There certainly is a sonic difference but hey you can't have everything.
Personally, I'd recommend the Beyerdynamic DT-1350 for a portable headphone in the aspect of efficiency and also physically - it's light, isolates well, folds really nice on your neck when not in use and you even get a protective case for it. Great for ride and travel. It also has some nice flexible "DJ features" like twisting cups and splitting headband. It's quite expensive and costs about 300 new and a little more than 200 used, but well worth it.