I tried 7 7970 with z87 g55, got 7 gpu with no error in hardware manager, but when I loaded up trixx the gpu had 0mhz core and 0mhz memory... So I couldn't get it hashing.
I Tried this setup using lbr amd modified drivers.
Have you enabled the Graphics Overdrive in the AMD CCC?
I have to deal with all three> AMD CCC, Trixx and MSI Afterburner for 3 5850 to setup as I wish...