Whilst reading Friday's Daring Fireball blurb on the subject, I surfed to the above table at Heis.de.

Two gotchas: 1.) the results are for unloaded signals 2.) the article is in German. At 0,37Ω, Apple's Lightning to 3,5mm Headphone Jack Adapter could be ready for sensitive earphones. Unfortunately, results for all devices and adapters appear to be volume or ADC limited. That is, unless the iPhone 6s measures worse than the iPhone 6.

Note: I've ordered the adapter, which I will test (loaded and unloaded: RMAA and square wave) on my iPhone 6, my wife's iPhone 7 and an iPhone SE (both scheduled to arrive in November).

Relevant links:
Original Heis.de article: iPhone 7 nachgemessen: Audio-Adapter liefert schlechteren Sound
iFixit's x-Rays: Inside Apple’s New Audio Adapter
RMAA: iPhone 6 24-bit