We have been to the Tetons and Yellowstone in September only once, in 2007. Your post prompted me to look at our pictures from that trip, something which is always enjoyable. Our last day in the Tetons was Sept 20th and the Aspens were showing good color. The Willows and other plants in the meadow below the Jackson Lake Lodge were very colorful and we got some good shots of a mother Moose with calf there. Obviously, peak color is different year to year.
The only negative for that time of year was that there was no snow on the mountains. IMO, snow makes the Tetons much more dramatic. Of course, an early snowfall this year could make all the difference.