I have a few MSI 56 cards that have crap Hynix mem in them and they all work with edited registries lowering all P-states. I have run memory voltage down to 850 or something and they ran, it was the clock speed mine were finicky about. They don't seem to mind any amount of undervolting BUT they will not handle the same memory clock speeds as the 64's or the one 56 I have with samsung memory.
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I own vega 64 with samsung hbm2 memory and it does all the higher frequencies but Im interested if hynix is more common on the vega 56.   Also if this is a launch card or part of later production, my initial impression was theres little difference between the 64 and 56 apart from bios limits maybe which can be changed.
If both cards have back up bios switch, doesnt that make it alot easier to update and keep a spare working bios just in case.

Really low power runs would be 120w or so but also can be unstable depending on temps I think.  Right now I have volts showing as set 975, 905 but some do 900,900 Im pretty sure