Efficient is a bad word too, because driver efficiency is not directly related to sensitivity. It deals with power transmission and loss - most dynamic drivers are *very* inefficient.
You're right, efficiency is not a suitable word either, because it is already used in this context to mean something else.
But if the distinction really that meaningful for headphones? When is the correlation between efficiency and sensitivity off? Are there some headphones that convert more energy into vibrations but get less of that to your ears? I suppose so, at least in theory.