I got 3 rigs with PRIME Z270-A mobos, and they all have 9 GPUs (2 are 9x1080Ti and 1 is 9x1070).
I have run all 3 rigs on both Windows (latest fall edition) as well as nvOC. No problems with either.
This is how I have my BIOS configured on these rigs:
Advanced/Platform Misc Configuration - Everything Disabled
Advanced/System Agent (SA) Configuration/Graphics Configuration/Leave alone
Advanced/System Agent (SA) Configuration/Graphics Configuration/DMI/OPI Configuration / DMI Max Link Speed = Gen2
Advanced/System Agent (SA) Configuration/Graphics Configuration/PEG Port Configuration / Gen2, Gen2, Auto
Advanced/PCH Configuration/PCI Express Configuration/Gen2
Advanced/PCH-FW Configuration/PTT Configuration/PTP aware OS = Not PTP Aware
Advanced/Onboard Devices Configuration/HD Audio Controller = Disabled
Advanced/Onboard Devices Configuration/M.2_1 Configuration = Auto
Advanced/Onboard Devices Configuration/M.2_2 Bandwidth Configuration = X2
Advanced/Onboard Devices Configuration/RGB LED Lighting/In Sleep, hibernate and soft off states = Off
Advanced/APM Configuration/Restore AC Power Loss = Power On
Boot/Above 4G Decoding = Enabled
Boot/Secure Boot/Key Management/Clear Secure Boot Keys = YES
Hope that helps.