Best advice I can give is majoring in a subject definitely helps... but what I think helps more is how much you keep up with it. I graduated high school, took a year off, and had AP credit for Gen Chem. So when I got to college, I applied to be part of a peer tutoring service we have on campus. I tutored gen chem my freshman year as well as other courses (while taking orgo). During my sophomore year, I tutored gen chem and organic. Junior year same. Also, I tutor physics, gen chem, orgo, calc, econ etc on the side for extra cash. Practice MCAT for PS was 13 and 14... hadn't even started studying. So the more you use it, the better you'll get at it and a lot of it will become intuitive as far as your thinking goes.
Quantum Chemistry will probably help you understand things in a greater depth (which will be beyond the scope of the MCAT)... and even then, the likelihood of you encountering such passage on MCAT isn't substantially significant enough that it'll make a big difference in your score. Looking ahead to see which courses you should take is good... but don't focus it on what you think will help you on the MCAT and in medical school. Take what you enjoy. Everything you learned in an undergrad course that correlates with a medical school course will probably be covered in a weeks time... after that, the **** hits the fan.
TLDNR:
1) Tutoring a subject will probably be more helpful than taking higher-level courses (combo of both is probably best though).
2) Taking quantum chemistry might not even improve your MCAT by a point because the parts of the MCAT that test on quantum chemistry isn't in as depth and you'll have already had it in gen chem.
3) Work hard on understanding the material in your freshman level courses instead of trying to conquer everything the summer before.
4) Take courses that genuinely interest you.
5) Medical school is harder that most of your undergrad stuff.
TLDNR TLDNR:
Not much. It's more math/physics with a focus on chemistry. A working knowledge of quantum physics is not required; you'll go over everything you'll need in physical chemistry (deriving hamiltonian, intro to quantum mechanics, etc.) Even then you get a little bit of it in all your chemistry courses (Gen chem, orgo, quant, inorganic, and pchem), just not in as much detail as quantum chemistry.