Interesting post as I tested a whole bunch of HDMI cables myself a couple of years ago. I was using my Oppo player a the source, but the changes you describe is exactly what I observed also. Plus the sound was actually the biggest impact, but only a few sounded much better than the rest, a lot sound had subtle changes.
My selection include similar ones, Wireworld, multiple Audioquest, QED, freebie, cheap from amazon. The best for sound and picture was actually the same cable, The Chord Company Epic 48Gpbs HDMI cable. Sound was the biggest jump over all the other cables, but also had the best picture. I have that cable throughout and not looked back.
Yes, I had seen differences like this going back to my Oppo long before the ATVX.. But my Plasma, wowee, these differences have become VERY clear.. The Plasma has issues of course, flicker and banding stand out most, but, it does not have processing like all these 4K displays and that lack of math makes for a very clear picture. ALl the modding I did to it took it to a insanity level. Of course its 1000% not piratical or even legal. I am very sure its vastly exceeding legal limits for RF emissions. I also cant put the back back on as it wont fit with the parts on it. And with 1000V all over it, its really dangerous.. BUT WOWEE it looks SOOO good, hahaha.. It is a REALLY good piece of test equip for me. I can REALLY see the picture and its not modified by any video processing or scaling.
What still just stuns me is watching things like "classic TV" on Pluto. Things like I love Lucy, Mission Impossible, The Andy Griffith Show, this kind of old show shot on film is just wonderful on the Plasma, but completly unwatchable on a 4K like my SOny A90K. I think the noise and film grain cause modern video processing to reduce the pic to ****.. The plasma just displays the moise and grain in the pic as is and this ends up being very pleasing to the eye somehow VS the **** that results from video processing a noisy pic. The problem is you cant get away from this math on a 4K. Even if you turn off everything and do all the right settings, the TV MUST still do moath to correct for things like the LED color gamut and gamma.
There is another unknown with my plasma mods. Lifetime. While I have not increased avg power draw and all I have really done is seriously sharpen up nano second pulses that fire each plasma cell with a tiny lightning bolt while making them far more precise and accurate,,, I do not know what this does to the life of the plasma panel. So far, so good, but I am keenly aware that I may have shortened the life of the panel..
SO.. I just want to make sure no one goes down the plasma path without realizing the issues. I need the best pic on earth for my work and I am willing to take the risks involved.
It has burned in.. What i know for sure is the pic coming from a ATVX is way better then you think using a 4K.
YES sound is a stunning difference with anything HDMI.. Its also the biggest thing that changes with the mods I do. The pic is a big jump, but the sound,, well,, its like a whole difference device..