i finally got all four of my r9 280x to work with risers (they have molex on them) but I am NOT connecting them to the power supplies.  I have 2 Corsair 1200axi PSUs - 1 is connected to mobo along with two molex connectors to this mobo as well.  4 cards all have risers attached to them - 2 cards to each PSU.  Finally working...each card is around 73 degrees..and hashing at a rate of 735 or so.  Windows 8.1 and the latest catalyst drivers along with SDK 2.9 I think.  REPEAT DO NOT CONNECT POWER TO THE RISERS - CONNECT POWER TO MOBO and to the TWO MOLEX PORTS ON THE MOBO as well.  I have a Honeywell fan blowing directly on the mobo and cards too.

Forgot to mention the risers are 1x to 16x; I am NOT using the 16x port on the mobo...