Efficient as in reducing the overall weight of a rocket to overcome materials physical limits. Am I wrong in thinking that bigger isn’t always better, the same way insects do not reciprocate the same damage as an elephant from falling from relative heights. I imagine though that the balloons, no matter the capacity of weight they would negate, would need to be detached within the first minute once the acceleration of the rocket out paced their natural lift. Again, minutely more efficient, but not worth the engineering/cost effectiveness.