How important is the efficiency of a speaker to you?

High sensitivity speakers w/ true bass extension necessitates large volume cabinets which can mean more $ & potential room / aesthetic placement challenges. That said, as John Atkinson often points out, high sensitivity drivers have substantially less distortion at higher output levels because of the much lower current flowing through their voice coils which generates much less heat & are “ working” much less hard. That said, some may not like the sound of the driver type used to create that high sensitivity ( horns, AMT’s etc). Every method has trade offs.

My own personal preference that to me sounds closest to live music is high sensitivity horns w/ tube amps but not “ flea powered” if you like to listen up loudly in a big room. Very difficult for just about any  lower sensitivity speaker at any price w/ even w/ heroic amplification to match the speed & instant dynamics of a quality horn loaded one. 
 

Volti Audio Rivals do it for me….

@ronboco 

As far at cost to achieve SET sound with power I have no idea.  If you're are really interested you should reach out to somebody like Pat Hickman at Whammerdyne.  He builds both "flea watt" and hi power hybrid amps and I'm sure he could explain both the benefits and drawbacks of the different levels of power.

Not very important. 

The best speakers I have ever heard, and the best I have ever owned (not in the same set, I could not afford the best I've heard), have all not been very efficient.

My old Koss Model 1a full range electrostatic speakers were 87db. But the clarity, detail, attack/decay, soundstage and image they produced are better than pretty much any high efficiency speaker I've heard. 

 I can think of a few exceptions, like the Acapella Audio Arts Hyperion, with their plasma tweeter, and their built in amps for the woofer section. 

Not particularly important to me.

The amplifier takes time to make this power (i.e., slew rate). So the amp "slews" 3.16 times faster to reach the same volume level resulting in increased dynamics for the high efficiency speaker.

that's not how it works, that is not how slew rate factors into how an amplifier works. Slew rate is directly related to bandwidth, not how quickly it reaches peak power operating within it's specified bandwidth. 

 an amp operating withing it's specifications that is  producing  100 W RMS at a given frequency will reach peak power in  the same amount of time as a 1 W RMS amp will.

Example.... at 1000 Hz the period is .001 seconds. So starting at zero crossing  it reaches a peak  .00025 sec later , crosses zero in the opposite direction at .0005 sec, the opposite peak at .00075 sec, and back to zero at .001... no matter how much power it is producing..  If it didn't it would be distorting the signal.