But none of your test deals with Rx 580 either so...
580s are knows to draw more power, but performs better in dual-mining than 4xx siblings.
Higher resale value too, plus you can't really buy 4xx cards anywhere.
That's problem of the new drivers, I'm not use them. Yes 5xx is better for dual mining, but they are better in power consumption too, but it need to use very low voltages and with those new drivers and only using AB it's not really possible.