Wow, I am impressed with this miner ....
2 Vega 56 and 2 Vega 64 (all are reference cards), from 8300 on V7 drop a little at 7800, using default config.
yes, that is normal drop 100 H/s cca per Vega.
But are 56 stable or not? Please post OverdriveNtool.ini file for cards, and what soft power play table you are using if rig is stable? Temperature core of the cards when mining for couple of hours?