Honestly, the battery issue was like when I first grabbed my Lumia 925 from ATT. It got about 8 hours which was kinda 'eh' and then after a few charges I was getting well over 24 hours. I had to stay over at my friends house one time and since I didn't have my charger with me, I set the phone on battery saver, locked it for the night, and woke up 12 hours later and it was on the same percentage that I left it.
Come 8.1 it was the same deal. A few soft resets to fix some problems, tinkering with turning location and cortana off, and boom, with location, cortana, etc. all on, my lumia is plowing through with over a day's worth of battery life again. I honestly think the battery just needs to calibrate itself; takes longer for some phones than others I guess.