For your GPU, 40-70 on max is normal. I had an R9 290x and I got about 45-60 on High settings. When you see it go below 30, it's usually the textures, shadows, and more of the world loading. It'll probably drop more when you get in to the larger cities, as I was gettinga bout 25-30 in the large cities; of course, depending on the population density you have set.
On any game though, keep ANY and all NVIDIA settings off, and you will see a great performance increase. It could severely reduce performance having even one of these settings on. AMD GPUs are not made for NVIDIA settings, as they do not have the appropriate chipsets, drivers, and software.