we were there in June for the first time this year from 2nd - 12th June. I was fine with the heat, though a little bemused / surprised the first time I went to sit down for the parade - that pavement was hot enough to cook with! My travelling companion was less comfortable, so much so that we only managed Festival of Fantasy twice, and with the help of parasols and copious amounts of water. I have never been in September, so can't give a comparison, but we were overall very happy. Hours are longer, so if you are happy with late nights, plan on being in the parks first thing in the morning, afternoons swimming or back at the resort when things are hottest and busiest, then back again in the evening.
we were looking at temps in the high 80s to high 90s, a couple of days were real feel 100 plus.