I think there was a discussion of this a few months ago here. I can't remember what the consensus was, but you can always search for the thread. There may be a correlation, but I think the best indicator of how well you will do is how hard you worked the previous 2 years, and how much effort and time you put into test prep. If you didn't put the time into the courses, you'll struggle with reviewing the material for boards, regardless of your mcat score. I didn't work hard in my first year physio course, and even though I spend the longest time on physio during usmle prep (6 days), I was still weak in the subject for boards.