Is VRAM not directly accessible by the system then? Since it's mapped directly into the CPU's address space, I had assumed that there's a simple DMA controller managing said access and I would then also naively assume that said controller is separate (or at least on a separate power plane) from the actual GPU.

While it may prevent sleep, one of the frustrations I have on an old skylake desktop with a bunch of 1080ti GPUs is that when set to power saving modes, the GPUs sit at 1x2.0 pcie lanes and low wattage. That slows down loading quite a bit.

So it will use more power, but nothing close to when the GPU is being used.