Is the Apple TV power cord worth upgrading? HDMI, Ethernet?

No, but people that read my posts know that I am against changing power cords. I have never noticed any difference between power cords - and I am not tone deaf! Remember that sound/video starts with a component's power supply - not with the  wire connecting it to an AC outlet!

For the other cables Monoprice is fine and low cost! Don't be taken by the claims of the golden ear moneyphiles! IMO the whole cable industry is just out to profit from the gullible with no objective proof of claimed superiority!

Higher quality HDMI cables do make a difference. Not sure you need to spend thousands. With regard to the power supply, maybe, but I have never tried, nor intend to. I run two Apple TV’s. Regarding PC’s in general, I 100% disagree with the above, but YMMV.

What about ethernet cable’s? Seems as long as they’re well insulated that’s all you really need to do.

I use cat 5 Ethernet it works fine on my 4K TV. I will try cat 7 to see if it improves the sound. I think the biggest change was upgrading to Netflix premium which opens up all kinds of data flow both video and sound.

Recently I went to cat 8 cable to my Apple TV, and I do think it works a little better. I also added a SR purple disc to the HDMI cable and that in itself helped project an improved image on my 85" Samsung.

ozzy

AppleTV might be improved with an upgraded power cord, but it still uses an internal switching power supply.  This is the main limiting factor on these streamer boxes. 

If you are willing to look at an alternative to the AppleTV, my recommendation would be to buy a Roku Ultra streamer.  Then get an external 12V linear power supply and ask someone to custom make a DC cable with 1.7mm x 4.75mm DC plug for the Roku.  That will significantly improve things (both audio as well as video quality).