Ok testing with vega 56. It really seems to be a power limit problem. With no real tweaking, just power limit on a stock 56 GPU, I get.
190w -19pl, 1300
235w 0pl, 1450
So, yes PL + 50 will get you good speed...but at an ridiculous power usage. This is stock 56 though, not soft modded.
Agreed, my watercooled Vega64 at stock clocks takes 500W to show 2200KH/s, it's crazy. Though it takes similar power in dual mining too.