Optimized today a RX570 Nitro+ 4GB - got 29,5 MH/s with the Glaymore miner under W10 and BIOS parameters: 1150GPU/2060Mem/0.9V Mem/0.86GPU with Power consumption @90-100W.
I agree on Mattthev notice getting more out, but with good knowledge of certain VRAM behaviour and long test scenarios - but possible.