FP91G posted this spreadsheet: https://docs.google.com/spreadsheets/d/1YXLubTptyI9n_dTNvuPBvqrjKWAx36DbhYj2H7-atcM/edit#gid=506857257

There's AMD section where you can find the hashrate for RX 580 4gb and 8Gb models

RX 580 4Gb - 720 Mh on the pool
RX 580 8Gb - 850 Mh on the pool


Nvidias are more effective on this algo