Just saw your post, I get it that you were using stock bios cards with Claymore and applying straps and rxboost to increase the hashrate. AFAIK Phoenix miner currently does not support "straps" for RX580 (all polaris) cards, so you'll have to bios mod these cards manually (using 1 click bios mods or any other tools) and then try using the overdriventool settings and then you'll get similar hash rates on RX580s.
I flashed the cards with a new BIOS and I now get 31.2 Mh/s on the RX580.
Thanks for the Steer on not using Straps!Seems to be mining fine at the moment after I tweaked the clocks a little. One thing I did find though is that if PhoenixMiner hash rate on any card goes to 0Mh/s (due to too much overclocking), when it restarts, it can't seem to do anything with the card. It connects and 'sees' the card but it is ignored from the autotune and restarts at 0MH/s. The only way round I found was to reflash the card BIOS...very strange. Is that normal? Or is there some other way to tell PhoenixMiner to forget it had an issue with the card previously and to start from scratch again?
Very pleased that there's an alternative to Claymore and, it seems more stable (apart from the issue mentioned above).