I can only coax the cards into giving a little more voltage by setting power limit to +50 and setting low GPU temperature card.
Keep the card cool, use +50 power and the card adds a little more voltage automatically. Once you've coaxed it 6000MHz is stable, but not until the card has warmed up and started increasing it voltages within the set tolerances.
Elpida 5700Mhz is the limit, maybe 5800 if your lucky, Hynix is better for sure by 200-400MHz.
These cards are crying out for custom/voltage control, I hope software comes out, otherwise we shall have to wait for customPCB designs from AIB's before we get voltage control. I hope software comes out as I feel the card has way more OC potential with a few extra volts, even on stock cooler, put it under water and these things will absolutely fly.