So 825 h/sec on monero earns about as much as 29 mh/sec on eth, but monero draws 3/4 the power. Monero seems like a no brainer for Polaris.
RX 580 is around 98 watts for 29.5mhs on Ethereum with an undervolt to around 895mv.
RX 580 is around 122 watts for 30.0mhs on Ethereum with an undervolt to around 950mv.
RX 480 can get a much lower power draw with undervolting, than the RX 580.
You would be wrong about the power draws for Ethereum.
Maybe you have some exceptional silicon? Most of the numbers I've seen on these boards are closer to 120W at the wall for rx580, i.e. about 750W for an ordinary 6-card rig. Seen plenty of people pulling 800-900 watts, probably bc they're not undervolting enough. My 580s are undervolted to around 850-860 and they pull about 115-120 from the wall while doing 29-29.5 on eth. They only pull about 90W on monero while doing 825-850 h/sec.
In any event, even if the magnitude is off, what matters is the relative power draw between eth and xmr. Even if the 580 is 98W for 29.5 mh/sec, xmr is better to mine at 75W for 850 h/sec.