ga z170x-gaming gt

I'm about to receive a nice freebie from a family member, two evga gtx 1070 SC's. I'm using a GIGABYTE GA-Z170X-Gaming GT motherboard with 3 PCIE slots arranged in descending order: x16 , x8 , x4. Will it be worth it getting an SLI bridge for this combo? Or will it default to x8 since...