Check DMM wiki, maybe they have numbers there now. They didn't back when I made the encyclopedia in Sanahtlig's toolbox, like what, eight months ago or something. But maybe they have them now?Yes, they get her passive. So more stacks is better. And remember that she doubles the effect on herself, since she gets a second copy of her passive.
Also to be noted is that she will shine in raids. Since the debuff is shared for everyone in the raid, the more people bring her the better she gets.