
AFAIK the GPU to system memory ratio is more relevant to 3D rendering than to color grading. The main reason for it is that GPU rendering systems mostly work only within the GPU buffer, because GPU access to system memory is suboptimal (read: slow). To get the best rendering performance you have to upload the entire scene into the GPU's memory, which isn't a big deal if it's small but throw in a volumetric cloud or a procedural ocean and your GPU memory overflows, and the renderer crashes.
But if your GPU is able to access 12-16GB it should handle 4K footage even with noise reduction without much trouble. I can run noise reduction on my system with its 16GB GPU with very little slowdown, at least as long as the footage is on an SSD.