That's great. I wonder if the Apple driver does nothing specific for the RX 580. My guess is that it keeps the power limit at +0, and it will force stock clocks (1340) regardless of your model, which it won't even hit consistently (more like 1150) due to the poor RX 580 voltage optimization. That's partly why the RX 580 performs "mediocre" in MacOS (mediocre means, closely same performance as 7950/680). The other huge part is really poorly optimized GPU drivers.
If you get the RX 580, I recommend +50ing the Power Limit, bump your clock to 1340-1360, and run an your mV @ 1060. 1040-1050 mV is very possible with 1340 stock speeds. I try to find the best for performance (to a degree) and don't worry too heavily lowest possible TDP. I'm not a miner, but a gamer. Finding the best of both worlds is fun, ain't it?
+50 power limit, 1360MHz clock, and 1060mV is more optimized for gaming. Probably a good reduction of 30 watts there (and much better, consistent performance). 1360MHz is held for all games or benchmarks 99.5% of the time. You may see a dip to 1350 every once in awhile, but it's rare and doesn't effect any benchmarks.
If you're OK with losing up to 1 FPS at 2k, then +50 power limit, 1340 clock, and 1040-1050mV would be a good target for the RX 580. Probably a good 35 watts reduction here.
Fan curves don't need to be touched. AMD has done a good job with this. Stock fan curves keep GPU temps in 70's range (with reference cooler). Thermal throttling won't even begin to happen until 90s are hit (I think), which probably won't ever happen unless you run your Mac Pro outside in the desert.
Either way the difference of undervolting the RX 580 is real. It's an absolute must if you use your GPU for anything at all. It's a win, win, win, win, and win, situation on all metrics.
Hopefully this helps. Check out my VR thread for more details related to gaming. Windows 10 benchmarks only though, since MacOS is unaffected....the MSI RX580 8G model does not have dual bios, and flashing a single bios card is a risky proposition. But I do like risks!
If you don't want to flash your card and spend most of your time in Windows 10, then I recommend using Wattman. DO NOT install Afterburner.