I actually disagree with you.
If you do a sub-I in September, which means you won't be done until the end of September or even November, it could be cutting it VERY close. Most attendings have to be reminded several times to write the letter and this can cause a delay of a month if the attending is super busy. That means you won't be uploading that LOR until early December.
I would highly recommend submitting your application as close as possible to when they open up ERAS to applicants. I say this because many programs start downloading applications the day they are allowed to begin following deans letters being uploaded. Basically, you can start uploadin your stuff on Sep 1st and the programs can start downloading the same day. Deans letters are uploaded on Nov 1st and this is when most programs will start giving out interviews. Some programs will actually offer interviews prior to reading the deans letter.
I actually made the mistake of waiting to get a letter of recommendation the second week in November. It actually caused me to not get two interviews since those programs didn't even look at the application until it's 'complete' (deans letter and all three LOR). By that time they had given away most of their interview spots.
I'm not saying it is going to break your application but, it could possibly cause you to not get an interview. If you know what you want to do and what programs you are serious about I would recommend you finding out if they require a complete application (all three LOR) before offering interviews. That way you will know.