I recently went from a 4080 up to a 4090 and for 4K it improved FPS where I wasn’t already CPU limited but also where I was generally getting great FPS with the 4080 in the same scenario anyway. In VR however it made a substantial improvement as in that I am mostly GPU limited so it gave me a great boost in overall performance in areas where I actually benefited from it.
As you say, CPU limiting is likely to be your worst case performance. Below is a screenshot on my system just now at KLAX, cloudy weather (live), FSLTL, A310 (CPU hog) and lots of MP traffic on my 5800X3D and 4090. This is without frame generation, where FPS would be double this, but you can clearly see that I am CPU limited by over 2 to 1, so a 4080 would pull the same FPS in this scenario. Your proposed CPU should be about 20-30% faster than mine so at least you would be in the mid 30’s before frame generation. Also, VRAM use at 8.5GB is well within 4080 VRAM capacity, although this scenario was done in DX11 which is lighter on VRAM use than DX12, but DX12 shouldn’t push it ridiculously higher.
