AMD Polaris cards (like your RX 580) benefit the most from changing memory timings and lowering the voltage. Both those things are done by editing gpu bios. There's plenty of tutorials about this on the internet since pretty much everyone does it. You'll get ~ 150-250 H/s extra on top of your current 550. But it's quite tricky with Mac OS, there's a good chance that the software needed for all this (polaris bios editor, driver patcher, control tool like overdriveNtool etc) is not available for your OS and is Windows only.
Ok I will look into partitioning some of the ssd to windows. I have no problems with windows. I liked it more than MacOS, but as I got older and needed something that just worked I ended up with Mac. Now that I'm looking into mining, a rig with multiple cpu and gpu cores on linux or windows seems to be what I need. :-)
Maybe monero is not the thing to mine. I see that I should be making $1.5 per day in mining and I'm looking at about 65c after 24hrs. My cpu mining is hashing 150-200h/s and my gpu is at 560h/s.
Does ethereum pay better? If this is the case I was looking into dual mining... I don't think I can setup dual mining using xmr-stak-amd? But if anyone has I would love to know! It looks like maybe I can modify some files to send ethereum hashes to decred.