OCTOMINER here. We work tightly together with a big AMD factory in China and we have received info that the RX 680 will only perform around 10-15% better than the RX580. Maximum 20% but currently it looks more like 10-15% increase in hashrate and efficiency. Also, around 10% retail price drop incoming for the RX 580 at the end of this month. We will post more here when I get more info and a sample in hand. 
We will try to be one of the first to also offer OEM RX 680 GPUs bulk to our bigger customers with already modded BIOS loaded onto them.
Currently we buy RX 580's BULK directly from the factory with already modded BIOSes, the GPUs perform better than Sapphire Nitro RX580 out of the box. Our OEM RX 580 8G GPUs get 30.5MH/S at 128-130W power draw, we noticed our RX 580's are around 5-10W more efficient than BIOS modded MSI ARMOR RX580 8G. (this is real life power draw, the AMD GPU power measurements from software are not accurate).
We will try to be one of the first to also offer OEM RX 680 GPUs bulk to our bigger customers with already modded BIOS loaded onto them.
Currently we buy RX 580's BULK directly from the factory with already modded BIOSes, the GPUs perform better than Sapphire Nitro RX580 out of the box. Our OEM RX 580 8G GPUs get 30.5MH/S at 128-130W power draw, we noticed our RX 580's are around 5-10W more efficient than BIOS modded MSI ARMOR RX580 8G. (this is real life power draw, the AMD GPU power measurements from software are not accurate).
30.5Mh / s at 130w? This is to slow and consume to much. Every of my RX570 does 31 - 32 MH @ 0.85 voltage. This consumes less then 130w. I will measure one rig in the next days.
I think yours are moded individually. Theirs are mass produced, so not tuned to the best.