It's based on the 1625 strap, since the 1500 strap wasn't stable for me beyond 1875.
That's very strange, I have several rx with hynix and the 1500 strap is the most stable at least up to 2050.
Maybe your cards have better cooling for the RAM? This is an Asus Strix. After tuning the 1625 strap, it was stable at ~27.2Mh with a 1950 mem clock. Now I'm testing 2000.
If it's a Strix you might have memory voltage control, have you tried/checked this?
I believe their custom VRM chip does control the memory voltage, but I haven't looked into it much. Since the RAM is not cooled (no thermal connection to the heatsink), I thought bumping the memory voltage would make things worse. The tuned 1625 strap is working quite well so far. It tested OK at 2000, and now I'm trying 2050.
Once I've found the limit, I may look into the memory voltage to control power use, and see if it is stable with lower memory voltages.
I have a Strix, pretty sure it doesn't work.
EDIT: Wait... wait... there's something here....