The game seems to be designed for 2GB GPU at 1080p and never exceed 2000 MB of VRAM with max settings. So if the game allowed extra VRAM space for GPU over 2 GB, would the game run with higher framerate, same, lower?
It would actually cache textures - instead of reloading them over and over and over - The game's streaming 'technology' is very console minded. Hopefully they will fix this and allow us people that aren't using a 3 year old graphics card - to actually use it's potential. No one would be able to say whether it would take FPS or gain FPS... depends on their engine.
I believe if the game used more VRAM, the thing that would be better is the popups, you can change it TextureMemoryBudget 800 is the ultra value, Nvidia reports that they didn't notice changes so far even with double values, other people say less popups, so far i haven't found solid info. Basically as i understand it if the game use more VRAM it caches more textures so the load happens faster as you move around, i don't think you would see improved FPS, i guess we should wait for people to dig deeper into these settings or for CDPR to give answers or some sort of tools.