I dont think you are planning too much and you could probably even add in one or two more cities. I did an 11 day trip to five cities (Shanghai, Beijing, Chengdu, Xian, and Urumqi) and it was quick, but not exhausting. Some of the places that I saw I wanted more time, but others it felt just right (if not too much!)

In terms of specific for your itinerary, I would leave out Xian. I was very excited about seeing the terracotta warriors and made sure to plan a stop in Xian, but all in all it was very disappointing and far from the spectacle it is made out to be. Its not that they were not interesting, but they were not even close to the eighth wonder of the world they were said to be.

OTOH, I would definitely recommend Chengdu (panda bears and a Sichuan food) and Urumqi (in the far northwest with a sidetrip to Tianchi - Heavenly Lake).