I have a hunch…that in fact the Sundara, HE4XX, HE400i and Deva all use very similar performing drivers, and it’s the pads that are the main differentiator between them. Just take a look at Oratory's measurements of them all below.
Sundara (original pads):
https://www.dropbox.com/sh/654wxtmhlq0dhqa/AAD75QfJfKih_8QL-a1O9bESa/Hifiman Sundara.pdf
HE4XX:
https://www.dropbox.com/sh/654wxtmhlq0dhqa/AAAB2SVYvAYTKwn44x3tcqZBa/Hifiman%20HE4XX.pdf 1
HE400i:
https://www.dropbox.com/sh/654wxtmhlq0dhqa/AAD2c-oE5YmNovJZLa_foGLZa/Hifiman%20HE400i%20(preliminary).pdf
Deva (wired):
https://www.dropbox.com/sh/654wxtmhlq0dhqa/AADPEKwIqSbIZcPdKwgHMiV1a/Hifiman%20Deva%20(wired).pdf
They all show dips and peaks in the same or very similar places, just in various amounts, and with different amounts of bass roll-off, both of which could be mostly explained by pad differences (pad seal being a major factor for bass extension of course). As they both use the same GRAS 43AG measurement rig, I overlaid Headphones.com's measurement of the Sundara with the new revised pads (thin green line), with Oratory's measurement of the Deva (wired, orange line):
These are very similar frequency responses. And I don’t think it’s a coincidence that it looks like the new Sundara pads and the Deva pads are the most similar of all the HifiMan pads, both having a ‘bulge’ (the pleather wrapping further around the edge to the ear-facing pad surface), and using the same materials. The only difference being the Deva pad’s slightly oval shaped inner ring that is not perforated like the Sundara’s, which could explain the minimal differences seen in the frequency responses above.
Now all this could easily be confirmed or denied, if either Headphones.com, Oratory or someone else with a GRAS 43AG has several of these different models, swaps the pads between them and re-measures. Until then I fully admit this is all speculation.