Stock didn't make me too unhappy in that department. Never measured it but it didn't seem too shabby. Yet as long as Sammy's stock is what it is - making an otherwise fantastic device feel slow and laggy - it's out of the question to return to it.
Running CM11 now. Makes me very happy as a whole but battery life is lacking. Getting around four hours of screen time with brightness set to the lower third of the scale. Also, I recently switched from Chrome to Firefox on all my computers including Android devices. On the 8.4 it burns through the battery like there's no tomorrow, often using more than 25 per cent. It doesn't on my other Androids.
Any experiences with Carbon, Clean or any of the de-crapwared stock ROMs?