I took comsae B timed, about 4 days before the real deal. I scored roughly 60 points LOWER on the real exam than I did on the comsae. My real score was still above average so I wasn't in danger of failing, but it did give me an inflated score.
I'm sure as hell glad I took the USMLE because when I used the famous bull$#!* official kirksville/AOA blessed comlex conversion equation, it underestimated my real USMLE score by 30 points.
I took the USMLE 12 days after the COMLEX and only studied for one day in between the two exams (brushed up on pharm the day before the USMLE). I couldn't sleep the night before the USMLE, so I was definitely sleep deprived. Luckily things turned out well and I met my goal score for the USMLE.
Perhaps I did better on the USMLE since my #1 source was UWorld and my annotated FA2008. The kaplan combank seemed like a waste of time, and the questions seemed too easy so I canned it after doing 60% of the questions. The interesting thing is that I did very well on the OMM type questions on all of my practice exams and the OMM on the real deal seemed straight forward.
Sorry for the USMLE rant, but my point is that I don't know how much stock I would put in the comsae's or BS kirksville/AOA conversion equation.
FWIW, I have a buddy that got a ~550 on the comsae B and somewhere around a 510 on the real deal IIRC. It definitely overestimated his score by about 40-50 points. However, he didn't take the USMLE.