We'll be there the last week of August this year for the first time. We've always gone on off-season before. I have all the same concerns about the FTP too, but....
Keep this in mind:
1) As someone already pointed out, the FTP is for only a percentage of the rooms, so that doesn't mean they'll be filling up the resorts, and in fact I called last week to see how much the FTP would be for my same dates at another resort, and I was told there was still availability there. So even a week ago they still hadn't sold all the FTP rate rooms yet.
2) The FTP is no longer being offered. That means that there shouldn't be any huge surge in August bookings going forward, at least not compared to the last few months' bookings anyway. With that in mind, Disney should have a pretty good feel by now how many resort rooms will be booked up for all the months of August, and if they feel they're exceeding expectations, they may very well increase closing times (well, for the MK anyway).
Personally, I think the worst case scenario we'll encounter is what others have experienced during the busiest August in years past, and from everything I've read, even the busiest isn't that bad, especially toward the end.
Jon