I don't think I am using that bay. at 4.7 ghz my 8700K idles at about 40c and hits around 80 in cinebench. My 1070 has fans going full throttle to keep it under 80 using sitting around 70c. I know 70c for a GPU isn't too bad but I'd like to give those fans a break.
I'm currently using the Cooler Master HAF 912 with 1x 120mm intake fan and 1x 120mm outtake fan and probably due to the hot components I'm using (ASUS Dual GTX 1070 and an 8700K) I feel like I can't get temps under control. I recently replaced my CPU cooler with a Noctua NH-D15 but I still feel I can do better. This case only has room for one extra 120mm intake fan and I'm wondering if I should just get a new case and if so I'm really at a loss as to what to go with.