Found this on youtube a few hours ago and though you all might find the results of interest.
I have sapphire nitros on the way to me now (few hours til touchdown), one of each 570 4g (actually i ended up with an 8G model) and 580 8g, so I can confirm these findings shortly.
Note: this video is in Russian, but the benchmarks are easy to see, he is using claymore.
The thing is that the difference, from my reading, between the RX480 and RX580 is nothing more than a change in the 14 nm process AMD is using to make the chip. Otherwise, they are exactly the same, i.e. no driver change needed, no VBIOS change (although some timings may be slightly different), etc. Design-wise, it’s the same chip.
Since you tested the 570/580 cards using the 17.4.3 drivers will the 16.12.2 drivers work on it? Asking since that seems to be the magic number for optimal performance on the 4xx series.