Yes, one of my users has VVX410s he got free from a law firm that closed and switched from Cisco SPA504G to those. I had quite a time getting them on the latest firmware but I figured it out and posted a howto.
I have a Grandstream GXP1760W that also works fine, and the HT802 works fine, but the HT814 did not works fine, and it’s a mystery. I like your diffing the confs idea. I don’t have any spare GXP2160s on hand but we’re swapping some out for Yealink SIP-T46U next week so I will test then and report back. I did see one website say it’s the extra headers, but that doesn’t make any sense.