I think you've misunderstood. You don't take an NBME exam at the prometric center. You take a computerized 50 question x 3 block (150 total) test which has the same questions from the free 150 released by the NBME. So if you've done those... well, tough luck, you'll be looking at 150 familiar questions.
The purpose of the exam is more to get yourself acquainted with the testing center anyway... once that's done you'll feel more at ease during the real thing.