make sure you all disable ULPS when in CrossFire/X mode... and using anything like MSI Afterburner, because ULPS enabled (i think by default) tends to power off one of the cards in CrossFireX which will crash out the whole system when applying settings via apps like MSI Afterburner
I'm not using C11.7
I'm crossfired and using MSI AB, when not in heavy use both of my GPUs drop to 157/300 if i fire up miners OC settings kick in without a problem.