This whole topic can be definitely proven on an optical bench. But otherwise, it's common sense. I can even tell when wearing a smudged or
cheaper pair of reading glasses that something is off, compared to a recently cleaned good pair. How much does it matter? Just depends. I'm
of the school of thought that these little things tend to add up. A minor compromise here and there, and eventually it really does show in the
print. But not everyone has the same priorities. What I categorically deny is that filter quality is not a factor. Every filter manufacturer knows
that! I could same about the lens analogy above: If you've ever owned a multicoated dagor lens with only four air/glass interfaces no way
your eyeballs won't pop out (I know, hyperbole- but you get the point), compared to a plastmat, not to mention some zoom lens with sixteen
or seventeen separate elements. I really really does make a difference, and if everything else in the workflow follows suit, even the public
will quickly perceive something, even if they can't explain it. Happens all the time to me. But a misaligned enlarger, or cheapo lens on that,
why bother....?