It’s impossible to accurately gauge such things. For example, the thing about boosting is that it creates its own feedback loop.
When a character leveling gets boosted, it takes away that character that would otherwise have been searching for a group to do the content instead as per usual. Yes, there are those who wouldn’t do it at all and just give up, but there’s no way to determine how many that’d be. All you can really assume is that there’s at least a small desire to level, since they are actually leveling.
So the ones not being boosted have a smaller pool of characters to form groups with, and so they get drawn into the option of being boosted too, and thus the pool of potential groups keeps being reduced further.
But yeah, I’m not a fan of all this boosting either. It’d be much more organic if they’d just reduce the exp gains by a lot more when there’s a much higher leveled character in the party when killing low leveled enemies.