Quick update, I noticed today that my high performing card (GPU 2) has SK-Hynix based RAM, the other three lower performers use Elpida. Not sure what this means for me going forward, but it's a clue. Starting to feel like a detective over here 
Input welcomed!
I think the Elpida RAM is lower quality than Hynix, at least that's what I've read somewhere on some forum. And lucky me (sarcastic) ofc I got the Eplida RAM in my GPU. Input welcomed!