[upofadown]

I think it depends on what you mean by "readability". Proportional squeezes more text into the same space. So you get more of it per eye fixation and as a result can read faster and/or exert less effort for the same amount of text. Monospaced puts everything in a grid and makes it easier to distinguish one character from another. So you get fewer errors due to the characters bleeding into one another. So mono works better when legibility is an issue.

So both have better "readability" in different ways. Faster/easier or more accurate. Which implies to me at least that mono is better for everything but body text even when things are ideal. I don't think it is possible to have a single proportional font that works for both definitions of the word "readability".