Ive fished a few tournaments in Sept on Sebago. Northern bass supply has been having an open that time of year on sebago for several years now. It certainly can be tough. I would catch a limit of smalies then try and upgrade with largemouth. It certainly can be tough that time of year for sure. If you get a 12-15 lb you will be in contention. One thing that you may want to consider when prefishing. They draw down the lake in sept. usually around 5 feet sometimes more. I grew up 30 min from the standish launch. If you want to do some prefishing in Sept let me know.