I do not think you are correct on that. Though I have not tested this in a long time, but from what I recall, there is a big difference when you get close to objects. The highres texture pack, which was automatically patched in through Steam and checked in my data files, shows a lot more detail when you get near objects. The way I believe it works is the baseline textures look similar at say 20 feet away from an object, but when you get near them, they become very grainy, as if they were stretched to fit the surface.
I recall it was extremely obvious, as in, you'd have to be blind if you could not see the difference, when you walked up to armor tempering bench. You could see a big difference on floors if you looked down, particularly wood.
As long as you have enough VRAM, there is no performance penalty to use them. You might as well check them out. At least the highres ones that Steam patched in. 3rd party ones may use too much VRAM.