Is there anyone out there using a RX-580 to plot? Can you post your plot speeds in direct mode.
for plotting the limitation you'll reach won't be from your card, with a 580 your speed will be far beyond the speed any drive can write, aside from an SSD, or plotting multiple spinning disks at the same time, you'll be limited by the write speed not the card.
I'd say you'll be dropped down from maybe 40k nonce/min to something like 10-20k depending on the other parts of the setup, when writing to disk.
You can check for speeds and best values in devices.txt here https://forums.burst-team.us/topic/58/gpu-plot-generator-v4-1-1-win-linux/186