i have 2x7990 + 4x RX 480 on the same mobo = 4GPUs + 4GPUs = 8 GPUs
When i run only 2x7990 i get ~208
When i run only 4x RX 480 i get ~210
When i run all 8 i get 350. Is there any reason for not getting ~420 when running all 8?
Possibly you have a weak CPU that can't feed 8 GPUs fast enough, I have the same issue with my seven 290x rig.
Try lower intensity, I find -i 0 is fastest on my multi-GPU rigs with slow CPUs.