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?