My 6 x R7 240 4GB rig is running at a total of 300W. It is 80W without my GPUs running. Therefore, I'm getting ~36W per card on average, but I think I am paying a price from trying to cut corners with an inefficient PSU.
I'd have to say that might be it. I've got an EVGA Supernova 750B on mine, and at idle it pulls 82 watts, and running 4 cards it's 182. With the other rig plugged in and running (4 x R7 250's), it goes up to 384 Watts. That gives me ~8 KH/sec between the 2 rigs, which should be ~1000 YAC/day when the difficulty settles down. We're not even down to 50% of the NF=14 diff (it will drop below 50%) and returns are back to ~800 YAC/day.
I've been meaning to do some extensive testing in the power-efficiency arena. Rough calculation, it seems I am costing myself $0.25 per day on one rig by using this PSU. But at this point, 750B might be an overkill for me. I'm wondering if crappy riser cables also play a factor...
Again, we can't predict exactly where the difficulty ends up right? It could be more or less when it settles. You assume the exact same miners from prior to the NFactor change. I say more, new miners will jump on board--optimism.