I've finally decided to try the regedit thing.
The 3 vega 56 there are all at 1408@910mV (haven't tried 905 nor 900 yet. that's the next step) and 1100@1050mV (V64 OC with V56 voltage), with min fan speed at 3500 and 0% Power Limit.
System is stable, have been running 24h, and had to restart it to test the new miner changes I did.
The device 0 is always doing more hash that the others, many times at 1920H/s. I suspect it's because that's the card on the riser connected to the x16, since everything else is equal. Other than that, it can only be wattman's sketchy code...
Did you control the resulted gpu voltage in hwinfo64 and wattage from the wall?
I think you should set "memory" voltage (it is not memory voltage in fact) to the same value as your gpu core voltage.