Contrary to what they told me when taking the MCAT years ago, it does not predict your USMLE Step 1 score. The rationale for taking the MCAT is that it is a good predictor of how you will perform on the medical boards. This is not true at all. It is very possible to not do well on the MCAT and to do well on the USMLE and vice versa.
It is possible to game the MCAT. At least it was when i took it way back in 2009. If you do about 5 thousand questions you will have seen every possible way they can ask those questions. Berkeley review and Princeton review had the best representation of the types of questions on the test. I saw physical science questions on the MCAT that looked exactly like the questions in the Berkeley review books. There were no real surprises on the MCAT.
With the Step 1 there is a certain level of uncertainty. A person can not prepare for about 30% of the test. These are often experimental questions. These questions cover things that you could not have anticipated. They are also purposely obscure questions that are not covered in review books. The medical board test writers do this on purpose. They do not want to know how well you can memorize a review book. They want to know if you have critical thinking skills.
So if you are struggling with the MCAT just take comfort in the fact that it does not indicate that you will be a bad doctor. Just do every question you can get your hands on multiple times and you will be fine on test day. So do Princeton, Berkeley, Exam Krackers and Kaplan question banks at least twice and on test day you will see repeats.