Depends on stats. If you are non-competitive, then yes it is a world of difference. 1) Because it takes far longer to get interviews vs. people with higher stats 2) By the time you get an interview you will now be up against guys who could not make it MD with beefed up stats. It is a domino effect.
The more competitive you are, the less of an issue it is. If you are competitive, then even an October application is fine. The only problem would be having less time choosing your schools and having to pay the deposit sooner (especially after December).