I already have 8 GPU working on identical supporting hardware and OS, aside from 11.12 driver, but this is also with radeon 5000 series and not 7000 series.
8 - 5000 series cards on the same type of Power supply as 8 - 7000 series cards is never going to work.
What Power supply is this your using that can power 8 7000 series cards?? and what are you using to plug them all in ...Adapters?...Duel power supplys?