i downgraded, i had to for virus Gengersense, but my battery doesnt seem to have digressed. what you should try doing is flash the update before the rom, reboot ur phone, thatl make sure it sleeps (thats what i read, may apply to w.e rom ur using) and then going ahead to flashing your rom.
(do at ur own risk im not responsible lol, i did it that way in mines and it worked perfectly, thats why i thought i should share it with u)