Just FYI; allocated memory doesn't necessarily mean
needed memory. Some games and even the driver can allocate a bit more than is strictly needed.
They way you check what's actually needed is through benchmarks, especially frame time consistency. Once you have too little VRAM, you'll get stutter, as the driver has to swap data between VRAM and RAM. When you are approaching the limit you will start to see occasional stutter, and when you push beyond that it will get pretty unplayable very quickly. It's not like you will get a 5-10% drop in performance, it will be far more severe than that if it's actually out of VRAM, in some cases you can even get glitching, texture popping even missing textures.
PCIe 4,5,6… will not help with too little VRAM, the problem with swapping is latency, not bandwidth.