These wont be viable for mining...Your probably looking in the 1K+ range per card with HBM2 16GB stack...and eth mining in the 40-60 MH range. You can buy 5-6 570s for the price of one of these.
It's a curiosity like the R9 295X2 or Fury cards. They weren't made for a budget user but it looks interesting and I might consider adding this to my card museum. They're not budget cards, again, I just want to know how Vega will perform, and this first Vega card will provide a good look into how the rest of the card range does.