yeah, I had this happen to me too, mainly in areas where there were many physics objects loaded or AI's and such, I was having mostly similar FPS to your 1080, (when I was using an r9 280) and I had to switch to lower AA and that fixed things, so I think that the game is more optimized for AMD somehow. But also, what is your CPU and RAM?
when I upgraded to a 390, things got much better and I was able to play max settings with nothing going under 45.
EDIT: I also think that Nvidia's drivers need a little erm... 'fixing' if you know what I mean (I have a laptop with a gtx 950m and it does the same thing)