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.