I would note the question isn’t should you add more rocket fuel instead of burning the structural mass, it’s can you convert the necessary structural mass into

I would note the question isn’t should you add more rocket fuel instead of burning the structural mass, it’s can you convert the necessary structural mass into impulse vs being dead mass. You need the structural mass one way or another, and adding more fuel will add structural mass. By burning the structural mass as it becomes less necessary instead of simply ejecting it you maximize delta-v.