Yes I have set it up using pci-e risers. Now I have 1080, 1070, 1060 and a 970 gtx all rendering together as CUDA devices and each GPU I add I would say render times drops proportionally.
Mobo is PC Mate b350.
I made a quick benchmark here for you with 200 samples (I would use 2000+ for the final render). The scene is an interior living room complete with lights, textures and hdr sky:
Just the GTX1070: 3m58s
1080+1070+1060+970: 1m26s