not sure 580 will change things significantly in performance per watt. More likely next generation Vega will though.
580 cards are the same polaris chip so until Vega we won't see anyting spectacular.
It is a swap from LPE to LPP, chips based on this process will purportedly deliver more processing power in a smaller footprint while sipping less power at the same time.
So the 580 will be more power efficient than the current 480 and slightly faster.
I'm waiting on a bios to see if it will be compatible with the 480 but I doubt it will be.