Same here, I have a mixed rig, 2 x RX 580 and 6 x VEGA 64, i am mining monero stable, but with same GPU settings, it crashes on Cryptonight-Heavy, also low hashpower.

(Edited)
I would like to add, that i've just tried cryptonight-heavy on a 6 x RX 580 8GB Hynix mem rig, and i get 5,3 kh/s constantly.