Are you suggesting that Scorpio will use all 8GB of memory just for graphics?
Well, obviously a double 480 or double 470 would beat the Scorpio in performance, but not nessesarily in 4k performance, you can say a lot about console hardware and obv. pc hardware is faster and anything from a 1070 and up from a nvidia perspective will be better then the scorpio but they balanced the AMD hardware out nicely.
Pairing two weaker GPU's togheter is always a "driver" and "optimalization" thing that never did well for me personally, I tried it a few times, but the bottum line is from a console perspective what Microsoft has achieved is a big step up from what the ps4 pro did, which has a graphics card weaker then the 470RX
And no it won't use the full 8GB's available for Vram, but my expectation is that in most scenario's it will use somwhere between 6 and 6.5GB Vram for the GpU and 1-2GB for the CPU for games so any PC equalivilant card to the scorpio should have the full 8GB to make sure it doesn't hit a treshhold because when Vram runs out FPS starts dropping. Now on the PS4 Pro side most 4GB cards will be enough since it only has 5.5GB available for games ánd the memory is also much slower.
Faster memory is tricky because textures load in- and out faster the amount of memory is a little "less" relevant because of effeciency in moving scenes, now again I am not saying the graphics card is "cutting edge" or anything we've never seen before but basically its a 480 teched unit with more processing cores running on a 1080's speed memory, which is something AMD simply doesn't offer on PC.
The RX480 struggles sometimes in 4k, but generally can run quite a few games in 30/4k native but the occations it cannot are possibly fixed by AMD by the customizations, you can see where the ps4 pro sturggles and many of those factors are cleaned out by the Scorpio.
Its going to be really fun to see how it does pair up with equavilant hardware from PC and see how it competes. Overall its a nice side-kick for my future pc and I am very happy with how it turned out, but this topic is mostly about price and that will be the buy- or skip factor for many games who are on edge on buying or not buying the console, but wether it turns out to be 400 or 500 buying a pc will proberbly not get you the same 4K results in games.