Needless to say, while battery life itself is extremely important in the long run of things, there will also be other accompanying updates with this particular update, including the relevant standard security enhancements that ensure your HTC One (M8)’s performance would be safer, while doing away with a slew of bugs as well as introducing general performance improvements.
I suppose most folks would just fall back on the Extreme Power Saving mode, where it will shut down unnecessary functions as well as perform tweaks to the other features that will help extend battery life. I suppose there is no harm at all in keeping your fingers crossed for the Android 4.4.4 KitKat update eventually.