Well, this is a scenario I was thinking through as well - ppt sending on Sept 10th and departure on dec 19th. If no additional backlogs happen, you should be OK (as I saw people waiting over 3 months and one week, I gave it about 65-70% chance), also keep in mind you will not have enough time to withdraw the passport if you do not get it by let's say Dec 15th.
Unfortunately, I had to wait for employment letter until Sep 29th, so my Xmas travel plans had to be postponed ...
I have a suggestion - it is not a good idea to frequently call DoS for status, it takes longer to get our passports back. Let's compile a list of September filers and choose for example 1-2 dates a month to call them and ask status for everybody in the same time.