I've done my own measurements, and leather pads generally have a bit flatter sub bass extension, but honestly it's not that noticeable in an actual listening session. I'd also never choose measured bass extension over complete tonality.
We can look at frequency response graphs, other people's opinions, etc. but obviously we're never going to know what we personally think of something unless we listen ourselves. And clearly FR graphs aren't going to tell the whole tale, as the shapes of these pads, driver distance, etc. reflect the sound in different and complex ways into the ear canal. In terms of blocking the driver with a rectangular opening though, I just can't see how that, and creating more internal reflections, could be a good thing.
The Audeze vegans were what was being recommended a few years back - they still have the best soundstage I've heard in terms of the HE-500, with good bass extension, but the treble is muted and they also sound rather muddy.
Also, just want to clarify that I'm not a treblehead, quite the opposite. But when I tried the Ether angled for a few weeks, and switched back to my HD 650, the more dark-leaning 650s actually sounded more resolving overall, as well as having seemingly more perceived treble presence.
For me personally, I just don't want that type of signature from my HE-500s. If someone were to want a sound somewhere within the realm of the stock velour, I would not recommend the Ether angled - I'd recommend the Focus Pad As. But this is all just my opinion.