I was one of the people who went back to iOS 9.3.5 to see if it would get better battery life. To some extent, I did, but, it wasn't as much as I thought. Both OS' were set up as new, and not restored from backups, so it's as fair as possible. Usage time on 9.3.5 was around 30 mins more than 10.0.2. Not a whole lot better. I was getting around 7 hours of usage on 10.0.2. With 9.3.5, I was getting around 7 hours and 20 mins. For all the new features you get with iOS 10, 20 mins seem like a fair price. If you're experiencing FAR worse battery life on 10.0.2 compared to 9.3.5, then try setting it up as a new iPhone. I always set up my phones as new. Never restore from backups.