I wouldn't be surprised if RX 590 can handle 200MHz higher core at the same voltage even while undervolted. This means that 1400MHz @0.9V or 1200MHz @0.8V is possible, which brings ~15% higher hash OR ~10% lower power draw on core-bound algorithms, including cryptonight. Ethash hashrate will most likely be unaffected, so this card wouldn't be viable for ethereum.