I have used ViperOne ever since I got the M8 and I must say that although I absolutely love this ROM and keep going back to it every time I flash another ROM to test it, the battery life on it is not good. That being said, however, I've noticed that the battery life isn't good on almost every Sense-based ROM I've tried on the M8, and I noticed that after I recently flashed Lollipop GPE which I am getting amazing battery life on, especially with screen-on times, but the weird part is that it drains while the screen is off (while I am sleeping) really heavily as though the screen is actually on, and the reason is still beyond me.
I've tried ARHD, ViperOne, and Renovate and got about the same battery results with them (both with and without ElementalX) so I am beginning to think it's really all about Sense draining more resources than GPE. AOSP has the worst battery life from my experience so I always use either Sense or GPE or both with MultiROM. Right now my primary is Renovate with ElementalX and secondary SkyDragon Lollipop with ElementalX, and of course I get much better battery life on the latter but better smoothness, speed, and overall performance on the former, so it has become a matter of performance vs. battery life for me.