Thanks for the digging, it does seem there was a die shrink then for the RX 590, which I vaguely remember. I just wasn't sure. But that helps to be sure. Still, I do kinda recall them being pushed farther then optimal to meet performance targets.
As for Vega OC...I am not positive on this as it was a while since I had one to play around with, but I recall finding some tweaking threads on OCN that basically explained, yes, you could adjust something called HBM voltage or something, but that was kinda mislabeled, and was more of a voltage floor for the chip than an actual voltage target for the HBM. I do not exactly recall how or if changing the memory voltage was actually possible. Again, it's been a while, so I guess worth looking into again, certainly now with newer drivers.