I had successfully run 7x RX480 8GB on Z97 Gaming 5. I use older driver like 16.9.2 or tried 16.11.x. Set at bios, 8x4x4x, Gen 1.
Extra thing I did was using Window Testing mode on by below step:
Open Command Prompt , enter bcdedit.exe -set loadoptions DISABLE_INTEGRITY_CHECKS and press Enter. Then enter bcdedit.exe -set TESTSIGNING ON and press Enter.

Hope this help.