[fritsch]

As for SD/720p upscalling quality, I would be also curious.
Also: is there a "whitelist" native auto resolution switching option in MrMC or on any other player on ATV 4K? I mean also for SD and 720p? Or the res is always stuck at the GUI's res?

The upscaling on the Apple TV is really good, but there's no auto resolution switching with the ATV4K and there's no white list (kodi's the only app I know of that does this weird white listing instead of automatically switching). The only switching the ATV4K does is refresh rate switching and color space switching (the most important one in my opinion right now). 

 Kodi autoswitches if no whitelist is set - this was changed ~3 weeks ago so that users don't have to configure adjust refreshrate in two places. Btw. the whitelist was a long wished feature by some "loud screaming" users ... you will find some funny notes on github concerning that.

So by default: v18 will search the current resolution for best matching fps. If you play a video with a higher resolution than your current resolution kodi will upswitch if matching fps is found. <- v17 behaviour.