I've gone the last two times in late September and it has become my preferred time to do Mt. Whitney. However, you have to keep and eye on the weather and you will need to bring some extra warmies.
3 years ago, you could not have asked for better weather. All 4 days in shorts and sleeping under the stars, with few folks. 5 years ago, the weather leading up to our trip with frigid but it warmed up for the 2 of the 3 days we were out...the last was a day of single digit temperature on the summit with high winds; however, it was in the mid-60s when we reached the Portal in the early PM.
The pack will be a touch heavier but wilderness will be lightly traveled west of the Sierra Crest.
