To put it all together, If one chooses to put a Vega in a 2010 Mac Pro which would require 2 8 pin adapters, I'd limit it to the Vega 56 and I'd under-volt, under-clock it for: Lower noise, lower heat, and reduce the risk to the motherboard. It's really a shame that there are no direct cables from the 900+ watt PSU to the PCIe slot area.
My observations with the MSI RX580 so far convinces me that I won't be buying any more of the 5xx series. They only seem to be a good option on motherboards with IGP. A "nerfed" VEGA (using VGATab) is still a much higher performer than the 5xx series. I would also seriously consider the WX7100 when it's price drops below the $500 mark. I can't talk about the lower WX models because I haven't used them and they aren't specifically listed as working. I paid $150 for a used W7000 and I would buy another one of those if I could not afford a Vega.