Mario Kart 8 was a pretty game when it was on the Wii U. Have a guess how nice it looks at full pelt in 8K.
A video of Baby Luigi nipping around a couple of tracks was uploaded recently, with the user playing the game through the latest version of the CemU emulator (1.9.0c). Running on a i7-6700K (clock speed not mentioned) with a GTX 1080 Ti, the footage is supersampled from 8K down to 4K, which helps because 4K is the maximum playback resolution for YouTube anyway.
We've already seen a bunch of Wii U games running with 4K graphics packs on an older version of CemU. The emulator has been optimised a bunch since then, as all the progress videos for Breath of the Wild has shown.
For what it's worth, the action seems relatively smooth. If you watch closely you can see the odd frame rate drop here and there, but the motion is pretty steady regardless. No word whether it'd hold up as smoothly under splitscreen conditions, but the fact that an emulator can happily play something in 8K at all with consumer grade hardware (albeit top of the line, if not close to) is neat.