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.
580 is NOT the same chip - they are making the 5xx series on an updated process that is a little more efficient than the process the 4xx series were made on.
Looks like about a 5% clock speedup on the GPU as well, no hard numbers on the efficiency but I'd guess 5-10% more efficient
Small actual upgrade, not to be confused with VEGA (which appears to be aimed at being the next-generation of the Fury line anyway).