No need for a sweatshirt. In fact, last year we went in mid-November and brought pants/sweatshirts and didn't break them out once.
We are from upstate NY and found the heat of September took a lot out of us. The only way we could combat it was to hit the park at rope drop, break mid-day when it's the hottest/most crowded and relax in the cool room or go to the pool, and then go back out at night. The heat made us exhausted.
Another tip we learned the hard way - bring extra clothes. We'd shower when we came back mid-day (almost necessary, even if you showerd in the morning) and did NOT want to put our sweaty clothes back on. We ended up wearing two outfits per day.