you're obviously running a mining rig. I don't know the answer to your question, but am interested as to how you've set up!?
So you must have the riser cards in the 1x slots? Have you a pic of the setup, that would be really cool
Looking at this :
https://www.pcgamesn.com/amd/amd-vega-mining-boost
it appears there could be a huge boost in MH/s, going to Vega. But the power draw increases are huge. I guess for a miner that's something that has to be balanced well.
You must be going very well with 6 x 580's. I would be having a headache, with the power draw and cost of swapping out the 580's with Vega 64, but I guess you have to speculate to accumulate!