I have heard, disable some devices in BIOS. That is pretty easy I will try that.

Some claim that Windows 7 can handle 5 GPU. Change to Windows 8 or Linux would take a while.

I guess I can also try different slots for the 1x adapters. This motherboard has 4 1x slots and 3 16x. I do have an ivy bridge cpu so in theory even 7 GPU is possible.