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:
http://imgur.com/a/IAFv6
Sapphire Rx580 Pulse 8gb Hynix
Proof:
http://imgur.com/a/IAFv6
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.
I have Sapphire Pulse 580 8GB with hynix and i am ok with that cards, They are getting 29.6 MH without problems. Now i have only 2 in work the temp are ok, 62 and 58 degree. I have made some mod on them. Both with the pc are getting 340 W from the wall. So its not a bad card at all. I am happy with this model. You can check the pic for more details
http://imgur.com/6QgVt0x
Maybe my sarcasm wasn't clear enough, try this one http://imgur.com/a/IAFv6
rx580s by sapphire are amazing and people need to stop bashing them because there were a lot of issues in the past.