I want to read a study that compares what readers estimate for much effort was put into producing the same page of text in two contemporary and basic serif and sans-serif fonts. My hypothesis is that the serif font is viewed as more polished or refined, and therefore the result of more hours of work. But I could be wrong.

This is in-line with the advice here to use serif for long form and sans for short. When you're making signs and things like that, you don't have the repeated forms to inform your ability to interpret letters, so the serifs act to confuse readers, while in long form, they add flair, which could be more artistic and tasteful.