Problem with sapphire 7970 440kh/s

[xxxguillexxx]

Good day

my problem is this I have a rig with 6 GPU. I have 3 sapphire amd 7970, 2 gigabyte 280x r9, 1 amd sapphire 7950. the problem is that the 3 has 2 sapphire 7970 with dual dvi those are going very well at 720 kh / s but the other only has 1 dvi and several mini display port that only gives 440 kh / s concrete mind is this http://www.techpowerup.com/vgabios/118860/sapphire-hd7970-3072-120207-1.html
if anyone can help me for that like other'd appreciate

regards, D