I'd go with the CPU upgrade before the GPU as that 1080 can handle almost everything tossed at it except for Ray Tracing. If you don't play anything that uses RT, then you're not missing anything with that 1080 and even if your are, as you don't have it, you still aren't missing anything if it handles 4k at 120fps w/o RT.So my bigger question is what to do if you're still on a paltry four cores?
From my side - should I upgrade my 6700K or my GTX1080 when new products drop in the Fall? (gaming only use case)
I suspect even with the horrendous bottlenecking an RTX 4k upgrade will give more (if not smoother) FPS. Don't know if the fact I've got HT will be any comfort!