I've run stak on the 580s for about 12 hours now. Reported hash rate on Nicehash is not noticeably different, but the error rate certainly is:
Left is Cast, right is Stak.
There's a thread on Github about this. Psychocrypt claims its a Nicehash problem which is BS. Stak has only recently started working at all with Nicehash in recent versions, and clearly its not right yet. It works fine on other Cryptonight pools, just not Nicehash, so I'll go back to Cast for running on Nicehash and keep Stak in reserve if I need to switch to another pool. I'd rather pay Cast's 1.5% fee than Stak's 7.5% rejected shares.
For reference, best performance from my RX580s is two threads each with intensity 1152 & worksize 8. That gives 414H/s per thread, 1660 total which is identical to what Cast does out of the box although admittedly at 10W less per card.