Most people (myself included) with Hifiman headphones have not had the privilege to test all possible earpad selections, so these opinions are often just due to a set of earpads sounding nice and the owner abiding by it, not from strict comparisons with 5 other sets.
Thus far I've only personally tested the Hifiman stock pl and vel pads, JMoney Beyer pads, and Audeze pads. My perception of other earpads e.g. stock Denon pads / O2 pads / Alpha pads are all still just extrapolation from others' opinions of them.
What is certain is that the HFM pleathers offer some unique characteristics that almost all other compatible pads do not have; the pleathers being shallow offer more gentle clamping, as well as a smoother midrange~treble transition, than other pads which are almost always thicker. Now this you can't mod to compensate for.
I know a guy who did extensive pad rolling here on head-fi, and ended up prefer the O2 pad to all other (but back then Jerg pad wasn't born yet). But then as you said, it's just second opinions. The only conclusion is that if there's a pair of O2 or Lcd3 pad for cheap around the UK, I'll get one to try myself.
I dislike the stock pleather pad, it's smoother but you lose some clarity and the sound become kinda muffled. I would love to see an FR chart of Jerg vs Velour vs Stock pleather for the He-6, similar to what we have with the He500.