Loudspeaker sensitivity and dynamics: are the two inexorably linked?

Not necessarily. My Apogee Acoustics Duetta Signature full range ribbon dipole planar speakers (fully restored/upgraded a year ago) are quite inefficient. To sound their best they demand a lot of current. When properly matched with components, particularly well matched speaker cables (Symo LS4X or their equivalent in production today - Artech Prism Time compensated cables), the dynamics are stunning and effortless across the entire frequency range. 

I like some high efficiency speakers for how lively they sound at LOW average volume.  That, to me, is more important than dynamic range—the ability to play at both extremes.  There are not that many high-powered amps I like a lot, in particular, I don’t care for most high-powered tube amps because they tend to sound hard and have an unpleasant “glare” (sharp upper midrange peak) I don’t like.  High efficiency speakers and the modest volume levels I prefer allow me to use the 10 watt or less tube amps I like the most.

Bobbyd,

“I'm not referring to maximum loudness or volume, rather that the music sounds less reproduced and more that the instrumentation and vocals are more real sounding through higher sensitivity speakers.”

this is my exact experience when moving to horns. I don’t see myself going back as I have learned that dynamics is what I was missing. 

Efficiency is simply how loud a speaker will play at any given wattage, but is measured at 1 watt. It has absolutely zero to do with how it sounds. Magnepans are not very efficient but sound glorious. 

Magnepan's sensitive and dynamic?! I like Maggies as much as the next guy, but those are not two adjectives I would ascribe to them. In fact, Maggie detractors have long found them lacking in exactly those two areas. They are in the range of 82-84dB, and require huge amounts of current to "come to life".