I have two VEGA-64 cards that can 't work on such high settings.
Settings of these cards:
GPU 1032 Mhz 905 mV. Memory: 1020 Mhz, 905 mV.
Consumes 170W GPU power.
0,97 g/s
Use AMD Mem Tweaker to apply weak settings on Vega Cards.
That should provide a bit boost to the hash rate.
Also, undervolt the core more as undervolting memory doesn't do anything.
CuckaToo31 is more of compute intensive algorithm, amdmemorytweak will mostly raise power consumtion rather than increase hashrate drasticly...