I'd strongly recommend against this current plan, for a couple reasons. The first is that it looks like you're applying to schools based on the MCAT score you think you would have gotten, rather than the MCAT score you actually got ("although the schools I am applying to almost all have higher average MCAT scores") which could be disastrous unless the rest of your app is stellar. The national matriculant average is 511 so there are dozens of schools you can apply to with lower MCAT averages.
If you are able to score a 519+, then your application does become stronger, but if you don't get your score back until mid to late September, there's a chance that a school will have already reviewed your application and made a decision (even if they don't tell you).
All of this isn't to say that you shouldn't retake your MCAT if you're confident that you can get a 519+, but I would err on the side of caution by applying to a few more schools with lower MCAT averages.
Kevin W, MCAT Tutor
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