Yeah definitely don't buy the Sapphire rx580's with Hynix, they're terrible cards with terrible hash rates, since they're the newest in the rx series they also have terrible resale value, the fans are terrible too and can't even keep the cards below 75deg celsius. Don't buy these, get some old (second hand) rx480s for double the price!
Sapphire Rx580 Pulse 8gb Hynix
Proof:
Sapphire Rx580 Pulse 8gb Hynix
Proof:
You expect that gpu to run at 50 degrees when dual mining?
You're stressing it like a madman, it's like you running up a hill while carrying 2 bags of cement.