Earlier quoted context omitted.
> Sorry. Thanks, but no need for the phony apology. Typefaces, like other graphic elements, might not be felt to the point of the reader explicitly "recognizing" these contributions. Their impact is real albeit subtle. Designers are aware of that but still work really hard anyway.
How can you demonstrate or prove that the impact is “real”?
If you have more resources and need to ask questions A/B testing doesn't handle well, you can also do it with good old-fashioned surveying. Take (say) a few hundred people sampled as representatively as possible from your target market. Randomly show half of them a version of the design that uses one typeface, and the other half a version that uses another. Ask them to rate how well they liked the design. See if there's a difference. Or ask them to recall things they saw, and see if there's a difference.