One potential issue for you is you seem to drain your battery completely 100% every day. So that of course uses up cycles and decreases the battery life. (Not that there is anything wrong with that as you purchased a phone to use it) but a battery looses capacity every cycle so potentially it will be less and less as time goes on if you do nothing different. I dont suspect the battery is loosing 1 hr of capacity due to a charge cycle so dont get me wrong. But on the flip side I also don't believe each iOS update is causing an hour of less battery either.
Completely opposite from you, I have had consistent battery life from out of box til today with 8, 8.0.2, 8.1, 8.1.1 and 8.1.3. As are others (most folks I suspect). So if it were an inherent iOS issue everyone would be effected the way I see it and that is simply not the case.
Really wish you could pinpoint what it going on for 100% but my gut is its not iOS.