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.
If we compare the memory bandwidth of the GTX1060 6 Gb GDDR5X (192 bits) and the Radeon AMD RX590 (256 bits), we see a 7% lag of the model from Nvidia, which is completely leveled by a higher memory speed (by 20%). Performance of the memory subsystem of both video cards is approximately the same with a small gain from Nvidia (when using ETHlargementPill).
Considering that GTX1060 6 Gb GDDR5X uses 16nm process, the energy efficiency of these cards should theoretically be worse than that of the RX590, which are produced with 12 nm process. But in practice, AMD cards can consume more because they use more cores.