While I've never heard about the direct board correlation before, here's what post-bacc programs with genuine med school courses (GT, RF, BU, Drexel) tell you:
-Taking med school courses in a post-bacc means you'll either get credit for them in med school (depends on the program and the eventual school) or will re-take them in med school.
-If you retake, you learn the material twice over. Your solid preparation leads to better grades and better retention of the knowledge. Better board scores would be the logical conclusion.
-If you get credit for these classes, you have a lighter schedule than the average M1 but the knowledge of how to handle the M1 schedule. You have more time to study for the boards plus a head start on the study methods the M1 year teaches you. Again, better board scores would likely result.
While I've never heard a program director claim better board scores (and certainly never read a study to the same effect), I have heard them claim a significantly higher % of AOA members, top GPAs, and hard-to-get residencies among their graduates. I'd assume increased board scores are plausible.
As for why more people don't do it: money and time. Why spend an extra 40-50k in loans and an extra year in school when you're already accepted to med school? The point of these programs is to get into med school. The time and expense of the grade/score/residency bonus just isn't feasible if you already have an acceptance.
