A competent team is never going over 10gigs of vram item. Going to the other pool for GPU is going to tank performance. The slow ram is used for non gpu tasks that don't use a lot of bandwidth like CPU audio OS ect. Beyond3d has a few great threads on it.That's the same thing like on a pc gpu which has a standard (usually low) amount of vram. If it is vram starved it gets help from the separate ddr3-4 ram of your normal ram.
So the series x is like having 10gb vram. But it can actually "borrow" some ram from the slower pool if needed that it will cost some speed,but what if the slow ram is used for cpu audio?