That 1150 for heavy is producing a lot of heat - I could see that getting temps up to 65ish.  Maybe your 950 setting for fast isn't being honored.  Have you checked (in something like HWInfo64) that your cards are actually running at 950mV?

Btw, I run Nitro 64s (air) at 1565 core, 1180 mem @ 920mV and get 1630H/s @ 160W for heavy (diff miner tho.)  Unless your power is free, and you don't care about the longevity of your cards, you might want to consider whether the extra 230mV is worth the additional 70+ H/s (it's clearly not, from an efficiency standpoint.)

And the 875 setting on mem is inconsequential - it is simply a floor for core frequency.  Mem is always running at a fixed voltage unless you changed the bios.