I've had a couple of the non-Ego "VV" batteries where you change the voltage by pressing the switch 5 times. Though I never actually measured the dropoff, they seemed to lose vape quality as the battery charge went down. At least with the juices / setups that really needed the higher settings.
I'm sure they can hold the highest setting for some period of time. But once the battery charge gets below a certain point, I don't think it will be able to "step up" the remaining voltage available.