We were also there late last September. The lines were rediculously low. We used two fast-passes; one for Test Track (cut a half hour line to ten minutes), and Big Thunder. The latter cut a five minute wait to a two minute wait. Wednesday in the MK this week, we walked on Splash Mountain three times, and I mean walked on. Other waits: ten minutes for HM, five minutes for Pooh, and five minutes for Pirates of C.
I agree that when you see a fast pass, grab it, but if you're going in late September, lines aren't something you should afford a lot of concern.
Always an exception: MK on Saturday. If you can avoid this, do it. When you spend a day or two getting used to walk ons, suddenly having half hour waits throw you for a loop. On the other hand, this will probably be the only day for Spectro and FITS. We spent the day in Epcot, then headed over around five, toured a little, and found our "spot" around 7 pm, where we had dinner and spent the rest of the night.
Pat