[phaedrus]

I have hyperphantasia. It's like having a CAD program inside my mind, and I can design entire physical devices, machines, or structures and later when I build them the 3D arrangement of the parts works out just as well in the physical world as my mental model indicated. I can also plan out algorithms for generating or slicing 3D triangle meshes in my mind, and when I write out the algorithm it works on the computer just as I thought it would.

I think "positive" demonstration of such abilities would be difficult to pin on the difference between individuals being just a "failure of language to compare experience." HOWEVER - I share your skepticism on the lack of demonstrability of the "negative" side of that equation in subjective experience. Let me explain:

I don't feel I have an inner monologue. Subjectively my mental process feels entirely nonverbal. Without other people around and a need to communicate with them, I only think in pictures and pure concepts. I can pull up a voice in my imagination, but it's much more like replaying a tape recorded message (complete with whatever environmental noise) than a narrative associated in some special way with my train of thought.

So I can understand aphantasia by analogy to how I myself once thought "the voice in your head" was a figure of speech. (And I did and still do think the Sapir-Whorf hypothesis is BULLshit.) But I should also be skeptical as to whether my conscious experience is actually totally nonverbal, or if I am just discounting things that are actually there or describe it differently.