My Rig has:
3 Sapphire rx470 Reference at 27Mh/s
1 XFX Black Edition rx470 at 26.5Mh/s
1 XFX 295x2 at 31+31.5Mh/s
on an MSI 270-A Pro with 4Gb Ram
The XFX needed a Fan on the Back for the VRMs and 2 x SP120 fans for the Radiator, Push/Pull, plugged to the motherboard and set at constant 100% Speed.
I had to undervolt through AB
Core Voltage at -70mV for GPU0 and -75mV for GPU1
Core at 1100
Memory at 1350-1400
And it still hovers around 71-72 degrees.
Next step would be to change the VRM Thermal Pads.

I have build another 3 x 295x2 rig with 1 XFX and 2 Sapphire. Same exact throttling problem, but only for the XFX, since I couldn't undervolt more than 60-65mV without crashing GPU0 every 3-4 hours. Sapphires are working like beasts at the same settings, without throttling and with -100mV

I never found any Bios for straps or Undevolt for any of them. I could surelly use one!!