The performance of the RX590 is increased by 12-15% (Core - 1469-1545 Mhz), so its hashrate during the mining of the Ether will give 30-32 mh, which allows to compete with GTX1070.
Ethereum hashrate has almost nothing to do with core speed, as long as it is high enough (around 1150-1200MHz mark). Increasing it further has no effect on the hash.
Every 100 Mhz gives about 0,5 Mh. Also you will have decreased consumption.