I have now adjusted vardiff to push up more agressively than down, that should help with this.
I just tried it out and it's not any better unfortunately. (I've left it for good 20mins also)
7850 @ 1GH = 5.38 MH/s (before)
7850 @ 1GH = 5.40 MH/s (now) (diff hovers around 0.010549)
7850 @ ZHP = 6.30 MH/s (fix diff 0.20000)
280x @ 1GH = 10.50 MH/s (before)
280x @ 1GH = 10.50 MH/s (now) (diff hovers between 0.022220 - 0.029832)
280x @ ZHP = 11.49 MH/s (fix diff 0.20000)