Everything is extremely variable and there is no 'correct' answer to your question. The groupings of low MCAT/high GPA vs. high MCAT/low GPA are not homogeneous. There are a ton of different reasons for each circumstance. Their performance in medical school depends on those reasons. However, the following may be of interest to you.
Just a little gorilla math to get started, being very general break down of the couple of medical schools that I've seen first hand or have been around:
80% - Doing extremely well
19% - Struggling, but doing fine
1% - Danger of failing
The first question is, what is success in medical school? To me the only measure is the quality of residency that one goes to/getting into the program that you want to go to. I'm being black and white (and it isn't my complete opinion, but it will keep things simple). There are only a handful of things that matter for residency applications. This is an approximate breakdown: Step 1 score > MS3/4 grades >>> LOR >>>>>>>>>>>>>>>>>>>>>>>> Research/Volunteer work > Step 2 score > MS1/2 grades. Thus, for comparison purposes, I would say Step 1 score and MS3/4 grades are the best things to look at.
First, Step 1. Step 1, if you don't know is the first of 4 national board exams (could possibly be changed to 3 soon). It makes the MCAT seem like a short trivial exam. It encompasses everything that one is supposed to learn in their first two years of medical school. Anatomy, histology, physiology, pathology etc. Who does well on it? Good students and good test takers. If you have the ability to buckle down, study when you are supposed to and have an aptitude for test taking, you will knock it out of the park. The people that struggle in general aren't 'not smart enough', but the ones that lack drive or the ability to focus and put in the hours. I think the stereotypical neurotic pre-med would excel at this.
Second, MS3/4 grades or clerkship grades. Clerkship grades are your grades when you rotate in the hospital. They are determined by two parts. Part 1, your shelf score and part 2, evaluations by faculty. The shelf is a 100 question exam at the end of each rotation. It covers the basics of whatever you are currently rotating on. This again is easy to study for and isn't generally a problem if you can focus and study effectively. The second part however is where many stereotypical students have some trouble. Evaluations are opinions. People will often complain that they aren't objective. My response is: life isn't always objective, and one's ability to be a good physician can't be measured purely by a test. Who does well on that part? People with good people skills, people who can communicate effectively, people who get stuff done, even when they aren't told exactly what to do. Theoretically, people with strong ECs should be those people.
It is my opinion, but I think I stand on solid footing when I say that this is one of the most important reasons to actually experience things in undergrad/pre-med and not do things purely to list them as ECs on your application. The number of hours you did things and the number of things you did is meaningless compared to the lessons that you learned. Patience learned from TAing, problem solving and self reliance learned from research, compassion from volunteering etc. Those skills are what make a good MS3/4 stand out and can't be faked or read in a book.
To come back to your question. Who will cope with medical school? Almost everyone. Who will excel? Good students who can also apply their knowledge to the real world.
