I mine with an RX 580, have one box with a pair in it. When the miners are updated it will take advantage of faster memory and outperform the 570. For the time being it is a bit less profitable.
I think that openCL applications will give the cards a new lease on life once the mining thing dies back down in the next 2-3 years as more and more cryptos jump off the PoW bandwagon onto PoS and maybe newer methods.
I wonder if you could share any links to info on setting up an RX 580 computer for mining? Are you using Nicehash or some other mining software? I'm about to try mining again on my gaming computer with a single RX 580 and I'm seeking resources. Thanks!