We're going sept 13-27, and were there aug 29-sep 12 last year. It was surprisingly quiet, hardly any queues on anything. Plus hurricane Frances came for labour day weekend, so on the fri we visited Universal Studios, a bit windy but deserted, and on the monday (labour day, frances came on the saturday!) we went to seaworld, wet but again empty. We also followed the quiet days listed in the Brits Guide, so don't know how much of a difference that made.
A great time of year to visit.