It does not seem to exceed the watt performance of RX 470.
Actually it should be better or everybody hope so 
if rx590 make better power efficiency it good news for miner.
but in long term when miner what to change card price surely drop very much because gamer like nvidia card more than amd card.
and performance of this card not impress when compare to 1060 nvidia will launch 1060 with gddr5x with same price and next year nvidia launch gtx2060 too.