On my Vega 56:

1550 at ~100 watts (power limit -50)
1800 at ~150 watts (power limit -25)
1950 at ~200 watts (power limit 0)
2050 at ~240 watts (power limit +20)

More power does not give better results because  further increasing the memory clock the card gets unstable.

Have a look at:
https://bitcointalk.org/index.php?topic=2256917