I have Gigabyte RX 580 Gaming 8G Hynix on Windows 7
22.3Mh/s at stock (1340MHz 2000MHz 1.15V 122W)
As soon as I modify the memory clock the hashrate drops to 20Mh/s
I strap at 1750 (not stable at 1625), I obtain 24.9Mh/s at stock and at 1000MHz 2000MHz 0.85V 74W
No memory clock possible without a hashrate's drop.