According to the AAMC, you only need an introductory level of knowledge of physics, general chemistry, biology, and organic chemistry for the MCAT. Some passages may describe upper-division topics, but correctly answering the questions will not require upper–division knowledge.
Nearly 50% of all MCAT test takers sit for the MCAT a second time due to inadequate preparation the first time, and many of those people are doing just fine in their science courses. Believe it or not, most students who do well on the MCAT spend between 200 and 300 hours preparing for the exam. Control the things you can control, namely your preparation for this exam. Trust us, you don't want to have to take it a second time.
Good reading skills are very important for the MCAT, even in the science sections. For example, humanities majors out-perform biological sciences major on every part of the MCAT, not just the verbal section! Medical school admissions officers actually weight the verbal reasoning section the heaviest of the entire MCAT, because they view it as a measure of a student's ability to learn and communicate.
While it's true that most very competitive medical schools look for a combined score of 30 on the MCAT, your score alone will not predict your acceptance or rejection from a competitive medical school. There is a wide range in scores that medical schools deem to be acceptable, and they will also look at your other application elements before making a final determination. If your MCAT scores are lower, you'll most likely need a higher GPA and a more substantial clinical/research record to compensate.