My money is on your HBM temperatures being your issue.

If HBM gets above 55C your performance gets hammered. I'm running in CAST with 1300soc timings on two different rigs, as my vegas just would not play nicely together (one card just will not work in any system with any other card in it!!)

I can see on the weaker water cooled rig that if the HBM/Core gets to 55C the performance drops from +2000hs to 17/1800 area. 

Do you know what temps your running? Are the higher performance cards cooler by some chance?

Also well done on getting all cards at over 1100 memory timings.