Sorry, but those are rookie numbers. You have to pump it up!

I get 815-820 H on all my RX 470s 4G (1275/1975) - while drawing 90W from the wall / card (i measured it - cause info from GPU-Z has nothing to do with actual power draw from the plug)
And on RX 570s 4G I get 850-860 H (1300/2050) - and I can get them to 880-890 (1350/2075) - but then some cards fail and restart, so i dont clock that high.
They are also undervolted, but i guess thats obvious for every miner in the business ... and it varies a lot card to card - some can live with -100 mV, some fail at -50 mV. NVIDIA cards are much better with undervolting - but not getting anywhere near these hash numbers (GTX 1070 can do around 720-730 - and its tragic, compared to price of the card and RX cards)

So - take your time, reflash bios on all cards - either find one, or modify it - and youll get much better numbers.