I've been experimenting too with the Gigapixel programs by upscaling them to 2x and I was able to implement some textures of skins and models ingame. To be honest, there IS difference and looks better.
I also found in another website that someone upscaled most of JKA textures x4 (up to 4k of resolution). For me it is an overkill if you are not playing on a 4k TV to notice the difference and the loading times (which it was pointed out by the creator of the mod) is too much if you are not going to be able to play it in 4k.
So, I have my personal project to upscale all the playermodels files from base JKA and from other singlepayers mods and play through them when I finish upscaling all of them.