Lol. I have a smaller battery than you, keep it on medium brightness, listen to music 5 1/2 hours a day, go on head-fi all day long, (and yes. It is a slower processor being a snapdragon s4 1.5 GHz dual core, but windows phone is much more fluid. The phone I have is faster than the original One and gs4... Android must really suck at power management lol. And those snapdragon 2.5 GHz quad cores must drain a crapload of power. Yes, my phone is slower than m8, but only a little slower than s5.) and still end up having 30% at the end of the day. I love it.
I wouldn't mind an M8 tho... It would be a nice phone. I'd be content with a Nokia or HTC. No Samsung or apple. They can die for all I care.