We were there last year the 8th-13th and will be there the 13th - 20th this year. My understanding is that there is little difference between the week we were there - and when you are inquiring about.
Crowds were non-exhistant (at least compared to what I read about summer crowds). Most rides were "walk-on" to less than 5 minutes wait. We only ran into a couple of attractions (Safari, and Test Track) where we waited about 25 - 30 minutes. We very rarely rode busses that were full (a couple of nights we did have full busses when we rode from the park at closing time). Now, we were there last year over the anniversary of 9-11, so that may have kept some folks away - but from reading other peoples reports from previous years.... September is always slow (as long as you don't go Labor Day).
I anticipate that this year the "Fairytale Package" will make it substantially more crowded with "resort" traffic - but that remains to be seen. I was told last year that ASMo was at 39% capacity when we were there. I am pretty sure it is sold out this year (we had to make a reservation change at ASMo & could not be guaranteed a room there - so we decided on Port Orleans Riverside).
I am still pretty confident that it will be one of the best times of the year to go - as far as crowds are concerned.
Remember however.... that is the HOTTEST time to go - and we had several storms roll through. Pack LOTS of sunscreen & bring ponchos at all times.