I think a lot of it also has to do with personal perception, as well. We went during the second week of September 2002 (the week of the 9-11 anniversary) and looking back, we don't remember one single time thinking that it was too hot. Our only complaint was the fact that the high humidity meant we were drenched with sweat when we played a mid-day game of tennis on the clay courts at the Grand Floridian, but still, it was far from too hot to play. We had fun.
However, every trip report I've seen from that same week is full of complaints about the heat, and how the heat affected people's mood. I honestly don't get that. I thought it might be selective memory, but we have over one hundred photos from the theme parks (digital camera) and not one of those photos shows us sweating or with anything but smiles on our faces.
So even though it's impossible to predict the weather, I do predict that if your group tends to whine about the heat, then they will think it's hot in September. If they tend to thrive in warm weather, then they will love September (especially with the low crowds and average 3 minute wait for headliner rides).