I think that in terms of general crowds planning their vacations around being there for a HalloweenTime 9/17 start date, it will probably not be too bad because kids are in school and it's an early start to Halloween season. A lot of people can't accept that early date as being HalloweenTime yet, and won't feel the need to be there until October.
But because it's the start of an official holiday season such as HalloweenTime, the novelty of it will probably attract more people to be there than there might have been if HalloweenTime were not starting that day. Also, in all likelihood, a lot of AP holders will probably come to the parks after they get off of work that day to see the HalloweenTime fun. (That's what tends to happen when Christmastime starts.)
I actually was at DLR for the start of the Halloween season 3 years ago - and it was another early start date just like this year will be. It was quite by accident - we didn't plan to be there for HalloweenTime, and we had already planned our dates. It just happened that HalloweenTime was starting while we were there. I want to say that it was not very crowded and that it was manageable, but I know that was largely because it was raining on and off that weekend. The threat of rain will scare people away. If there had been no rain predicted for that weekend at all, I'm sure it would have been more crowded.
October has a lot of variables that contribute to higher crowds in the first two weeks of that month and at the end, closer to Halloween. There are various school breaks and events that will make the crowds heavier - that and the fact that October just seems more 'Halloween-ish' than September. September doesn't have those same elements to contribute to crowds, so it's always a bit better.