I wrote this on another thread I think but I have experienced about the same battery life on all iOS releases from the iPhone 4, 5 and 6 from iOS 4.0-8.1.2! It all seems to boil down to if there was a stuck process or not which I've had happen on all devices and iOS versions at some point. That's the only thing that would dramatically alter my battery. Download one of those free apps that show your cpu usage and check it every once and awhile through the day to make sure your cpu is idling down properly. It's all about the race to sleep!
Sure they can optimize code somewhat but ultimately your device is always going to physically drain the battery the same with pretty much all iOS releases. So when I see all these battery life posts about one version making things way better it's just hard for me to believe based off of my expirence and usage! Im not saying performance isn't better (or worse!) though! iOS 6 on the iPhone 5 is still the best experience I've had as for consistent performance and keeping apps/safari tabs suspended in memory. But when I updated it to 7 then 8 I still same the exact battery as 6. It all depended on my usage. The thing about iOS 8 that I noticed immediately was that there is sometimes a half second to almost a second of a pause before an app will launch from the springboard after touching an icon. This was never there in ios7 or six. Things always sprang open with no herky jerky delay. I don't know if this is fixed or not on 8.2 because I'm currently enjoying my 8.1.2 jailbreak. But man that's annoying and it does it on my iPhone 6! My wife's iphone 5 on iOS 6 outperforms my 6 in basic tasks. Anyways thats a whole other grumble sorry...
I was one of those people who though my device was magically getting way battery life but over time I've learned it's not much if at all different. (For me anyways

(my 2 cents)
Oh and I turned my battery percentage off and now I try really hard not to turn it on and check it anymore because in the grand scheme it doesn't matter and the phone lasts me all day. I was obsessed with my battery life for a long time too there is hope people!

turn off the percentage and enjoy your phone!