you can clear all actual content with a team of 5* event girls easily, they are pretty strong, so no need of 6* girls for that, but if you are asking if 6* girls are worth it, yes they are, you can clearly notice the difference, even with just skill lvl 1 and only 1 equipment slot, their skill at lvl 1 is high already (besides they are all strong as fuck, alpinia´s 4.0xdamage to a single target+heal is gross) and their really good stats can compensate in some way the lack of equipment slots, so what i can say is, are 6* girls really worth it? yes they are really strong, are 6* girls neccesary to clear actual content? no, they aren´t, you can clear everything with even 3* evolved girls