It would be interesting if you could install a lesser card simply to gain more VRAM. Though the memory bus of the card you choose would weigh heavily on the added performnce, but it's an interesting possibility nonetheless. perhaps a memory addon card could also be a possibility since it can be addressed separately and only has to shuffle info across the pcie bus. Just a thought..
With the way storage works, if this "lesser" card has a much slower or higher latency access to the memory it will affect all of your performance.