Assuming that the source material was 8k and not just upscaled during production:
- Content has to be scaled down (either during production or on your HW)
- Content has to encoded in (lossy) codec
I'd assume that downscaling->encoding should yield better results than encoding->downscaling hence watching 8k on a 4k screen would be inferior to just playing 4k (if both were encoded in the same quality).
The peru video - I am curious if the new MBP 16" can play it at 8k @ 60fps HDR? You may need to install chrome to see the option for 8k: On a PC/Win10 with ~1650 graphics card it uses ~80% of the GPU but CPU stays close to 0%.