I think going SSJ didn't help all that much (if at all), considering they were already <= whatever power #18 was using in a giant costume, with no leg usage. So if going SSJ helped them that much, so they logically would not still be at disadvantage. Yet, they think this, without even attempting to fight her.
So all in all, if the costume is limiting their speed, then simply getting 50x more powerful isn't going to help them, the same way getting 50x more powerful wouldn't have helped Trunks land a blow on Cell if his speed stayed the same.
I would not have the boys any weaker than Piccolo, specially at how Green Man was surprised when they showed his full power at SSJ.