How important is the efficiency of a speaker to you?

Not particularly important to me.

This has been discussed ad nauseum here on Audiogon. Low impedances can lower sensitivity which can make amps work harder to produce power and increase amp distortions. The degree to which the amp distorts due to this depends on the stoutness of the amp. The greater the loudspeakers sensitivity the more likely it will be a more open gateway to what distortions are still presented by upstream components, including but not limited from the amp. What is often less discussed is that varying impedances can strain amps too, and is more likely to present difficult phase angles to the amp as well. It is far easier to present and maintain a steady impedance by lowering the impedance of loudspeaker than visa versa. Electro-mechanical components are much more difficult to make perform well than purely electrical components. The variance between loudspeakers is typically much greater than between decently designed amps. This might suggest that more leaway be given to loudpspeakers than amps. As has previously (and wisely) suggested here; speakers designers have to choose what paramters and associated compromises combine to best meet ultimate design goals. Some if not most of the best measuring loudpseakers have lower impedances, and ergo lower sensitivity. There are a bountiful number of well designed amps that can handle such loads with minimal consequence.