The MCAT is a strong independent predictor of medical school performance and Step scores, outperforming undergraduate GPA.
MCAT scores almost double the proportion of variance in medical school grades explained by uGPAs, and essentially replace the need for uGPAs in their impressive prediction of Step scores. The MCAT performs well as an indicator of academic preparation for medical school, independent of the school-specific handicaps of uGPAs.
Ellen R. Julian (2005) studied this question.