RX 580 makes about 750 h/s and x 2 this is 1500 h/s in Monero. 1 x RX Vega 56 makes 1750 h/s in Monero , power consumption is better and lower with only one card so the equation solution is pretty simple. 1 x RX Vega 56 is better than 2 x RX 580 cards, at least in Monero. Haven't had the pleasure to work with RX Vega 56 yet (I will after 1.5 week when I will build a 8 x RX Vega mining rig in ASUS mining expert board).