just tried running on 12*570 8GB - looks awesome!
never seen such rates on cryptonight on polarises.
there's something consistently wrong with two cards:

and with slightly reduced volts:

old GG and "slightly" xmr stak with two threads, 896 RI and 8 worksize for 4 GB
The funny part is, even though they pull 1k, with FAR less energy than ethereum, whattomine and many other sources have them hilariously underrated at around 700 h/s each!
the 4gb memory controller on rx 570/580 and the 2gb 560 (same memory codes , elpidas, hynix etc.) is insanely fast in xmr.
Many people will be surprised that, rx 560, 2gb without even 6 pin can do 540 h/s with normal PBE tweaking! i even got core way lower and they still hash at 540 h/s! $$ and energy wise, thats nearly better than VEGA!
use 1/2 the values to fill the memory of the 4gb ones (448 RI, 8 worksize, 2 threads) maybe xmr stak if you wanna experiment, but Old GG and new GG are ridiculosly fast out of the box on these cards)
Zawawa we love you

Try 459-460 RI. On 270X 4Gb it gives best speed.