Yeah there's def a variance here. Ref V56s flashed to V64 bios and true Vega 64s all seem to be straightforward to get hashing > 19 kh/s. I have a few things to test, but I need to get a few Vega 56s going that aren't flashed with the V64 bios.

Are you on win10 or linux? And, I assume you've tried configs like L24+24 and 28+28?