I wish I had some good data on the various ESL's I've owned. Acoustats claimed like Soundlabs to be 88 db/watt/meter. In fact I'd say more like 78 or 80 is for real with a heavy lean toward 78 db. Stereophile did measure 101 db/2.83v/meter which was lower than the claimed 105 db for the K-horns, but still awfully high.
The Klipsch discrepancy is especially problematic imo, as I often see them recommended to newbies based on that sensitivity. Go to a random audio forum, ask for advice on what to buy, say you like to listen loud, and that you only have an avr; chances are you're gonna get a lot of Klipsch recommendations.
For their little box speaker, Klipsch figures can be attributed to marketing. Anytime marketing gets involved, things deteriorate. That is not an excuse. It's just the way things are. However, my guess is that any Klipsch speaker is going to be a reasonably easy drive for most amplifiers. With this in mind, the current Klipsch company is not Paul Klipsch's company. It's a too familiar scenario--would Gordon Gow and Frank McIntosh be happy with their progeny?
When discussing sensitivity one has to weigh the lab with real life. I don't know how they measure now, but the old horn based Klipsch were measured in their anechoic chamber. Real-life 'in the living room' figures were always lower. But not by much. Richard Heyser measured 98dB SPL
at 3.5 meters, and suggested that 104dB at 1 meter was not an unreasonable figure (that was the company's spec). Heyser measured at 3.5 meters since in his opinion that was as close as you wanted to get to the loudspeaker in practice.
Also, SPL sensitivity is FR dependent. Heyser offered the following K-horn graph:
FWIW I have used La Scala II's with both a 12 watt/ch tube unit (6V6GT push pull) and AHB2 100 watt/ch SS unit. With either it will get too loud too quickly.
Regarding Acoustat. These were relatively difficult loudspeakers to drive regardless of SPL. They did not mate well with many cheap amplifiers of the day, and I went through several trying to find one beefy enough to drive them to decent SPL. I finally settled on their own MOSFET (Jim Strickland's design) 200 watt/ch amplifier. It was a long time ago, and I never had any objective data, but I suspect it was the amp's interaction with the wide impedance load, both reactive and resistive. Also, unlike horns (which just keep getting louder) the electrostatic panels reached a certain SPL and then seemed to just stop huffing. What I wanted was a cross between the Quad and K-horn. Somehow. LOL
These days, with cheap watts, the benefit of high sensitivity is (or should be) lower distortion as the drivers are not asked to do as much at a given wattage, and hence should not distort as much.
However that is, with these little monkey coffin shoe-box speakers, and modern amplifiers, it's mostly a wash. Not much to care about. Unless you are talking something from specialty days, like an LS3/5a with a Futterman OTL amp. Or if your idea of amplification is a SET. Then things must be considered.