I arrived at migraciones last Wednesday just before 8AM and received turno number eight for the prorrgas de permanencia, but that was also the day the subways were closed until 9AM. There still wasn't much of a crowd when I left at 8:45.
Given the fact that your entry date was the 15th, today was a holiday following a weekend, and tomorrow is only the "14th" I doubt you will have a problem renewing at migraciones. It won't hurt to try, will it? If they won't renew it for you tomorrow morning that still means you will pay the same $300 pesos when you leave, whether it is tomorrow or three months from tomorrow.
Please let us know what happens...as well as what time you arrived at migraciones, the number of your turn, and how long you had to wait. I'll do the same after I go again later this week.