Something tells me the answer is actually some really cheap disposable IEM or earbud. I really don't get why you're chasing ultimate efficiency when, if anything, people need the opposite to keep to sane volumes without engaging digital volume control. This feels like the early 2000s with everyone making smaller phones just because.
I wouldn't say it's a chase as much as a curiosity. And mostly for 30-60mm drivers for headphone use. I like exploring extremes, taking things too far and then dialing back to a sweet spot
Some of the most sensitive headphones (excluding IEMs) I've seen:
mW to produce 110 dB SPL @ 1 KHz
1.6 Denon D7000
4.0 Focal Clear
4.0 Focal Utopia
2.5 Focal Stellia
2.5 Sony 7506
2.0 SoundMagic HP100
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