How critically can you listen to headphones at a show like CES?  With all those people around, noise, choice of music/source etc.

Yes, most of the time, CES is quite noisy.

That's why they used hotel rooms instead of booths in open space. Good thing was there weren't many people when I visited HiFiman room (it was lunch time I think). I could listen to the headphones in rather quiet environment.

Only problem was I could not use my own sources... Thankfully several tracks (such as Eagle's Hotel California) that I was familiar with were there, so I could do some evaluations.

How critically can you listen to headphones at a show like CES?  With all those people around, noise, choice of music/source etc.

Hifiman had a separate semi enclosed booth on the main floor for the HE1000.

90db/mW is Hifiman speak for 84db/mW

90dB/W maybe?

It will be quite difficult to put an efficiency number on the SE-700 as the impedance varies with frequency and thus also the efficiency when using a voltage source.

I saw specs of the driver being 0.07pF but this should probably be 0.07uF.
Most piezo electric tweeters are between 0.1 and 0.2uF but these have to deliver much higher SPL even at 1 meter distance so assume 0.07uF is plausible.
When we consider this capacitance accurate and ignore the series resistance and measure the efficiency at 1kHz the 'impedance' would be around 2.2k Ohm and quite capacitive which is a difficult load for many amps by the way. A small series resistance of at least 10 Ohm is needed.
It will be around 220 Ohm at 10kHz and 22kOhm at 100Hz.

To get 1mW in 2.2kOhm (1kHz) you will need 1.5V
At 3V it will give 100dB so at 1.5V it will give 6dB less SPL so my calc (assuming 0.07uF is correct) says around 94dB/mW at 1kHz.
But as said you cannot really specify this headphone in dB/mW so for that reason it is specified in dB/V

More realistic would be to specify the efficiency at 1V (like a lot of other headphones) which would be 90.5dB/V
Most headphones are between 100dB/V and 120dB/V so 90.5dB/V is really insensitive (but not inefficient in this case).
The HE-6 is also around 91dB/V and is both insensitive and inefficient.
The DT660 is also around 91dB/V but is more effiecient than the HE-6
All 3 mentioned headphones will be equally loud when fed from a low output R amplifier though as only the dB/V number says something about how loud something is compared to other headphones.
the dB/mW numbers by themselves (when not considering the impedance) say nothing about relative loudness differences, only about efficiency differences.