The first thing that you have to do is to force gpus at a specific Powertune level. You won't hurt the cards, you will just draw some more power. Some cards are more efficient the the others at powertune level 0, but when you rise powertune to 10 or 20 the get equal. So, first thing to do is:

"gpu-powertune" : "10,10,10",

see what happens, then try also

"gpu-powertune" : "20,20,20",

And don't be afraid to do some overclocking ... E=1000 and M=1500 should run just fine at 880 kh/s.