Wow 8 hrs? You must not have a lot of accounts syncing then (Email, Google stuff, Facebook, Whatsapp, etc.)
2 Google accounts, and now my Hotmail Account through Gmail 5.0, and the rest like Facebook, WhatsApp, Google Plus, etc...
I think the key is brightness and keeping an eye on Running apps at least once a day plus the usual check on the Battery usage, that tells you when you have a troubled app, but I'm not on data most of the time, at home and work I'm always on wifi, so I'm on data only for about 3 hours a day and that helps on battery life.
I unplugged my phone today at 5:30am, it's 7:45pm and I'm at 50% with 3h45m of Screen on time.
Again, for me that's what I was expecting, I replaced my Samsung Galaxy S5 with the Z3, but I was getting no less than 6h of SOT with it so I was totally expecting this battery life on Z3 with my normal usage.