Same issue here.
Asus z170-A, 6600k, Asus STRIX GTX970, dell 2715h on DP, sony 40hx853 on hdmi, sys def is imac 17,1. Tried ssdt injection for gfx1 and a lot of different things, still can't boot with 2 monitors connected. I refuse to enable CSM since it makes my asus board to show all POST info...