lol, 7870 has similar performance as gtx 580 at like half as much watts and 7870 has huge oc potential so could easily beat a gtx 580 overall even if both are overclocked. gtx 580 is what 1-5% faster on some stuff but 7870 can be oc'd like amazingly to destroy it and still take less electricity. Overall performance equal as its to close to give a winner.
Well, the R7870 used roughly a 100 Watts LESS then that GTX 580. When we reverse calculate and measure the power consumption the R7870 uses roughly 130 Watt where the GTX 580 hovers at a 235~240W, and that's measured in game while it's peaking and stressed. So for the R7850... well we measure roughly 106 Watt. Amazing stuff really.
Read the question which was performance for the price
and the GTX580 is substantially cheaper!