Efficiency of speakers?

My experience has been the opposite.
I have always found that less efficient speakers sound more controlled and have the capability to produce a more convincing sense of scale and dynamics. A good sealed box enclosure (infinite baffle) tends to be very inefficient, but if driven with sufficiently high quality power the result can be extremely accurate compared to all but the best ported designs. Many of the high efficiency speakers I have heard sounded relatively out of control to me.

Thanks all,I should have clarified that I am only interested in loudspeakers using conventional cones, regardless of their material, such as the Salon & WP8 as examples.

Because you're interested in cone speakers only, my experiences are only half usefull to you since I own a hybrid cone/ribbon design. Efficiency is low, due to the closed housing for the woofer and the low efficiency of the ribbon. The low-end is among the fastest and best controlled I've ever encountered. At first you think there's hardly any low-end, but baby, there sure is. So my experience suggests that the lower the efficiency, the faster the sound is.