The passive reduction in both the Zulu 2 and Bose is not very good at all. But having said that, in the Cessna and Pipers and my own Jabiru that I fly, I feel that it does bring it down to safe levels. It's certainly still quieter than walking down the street in London.
One of the things that I don't like about the Bose is that it does not automatically switch on when the master switch is turned on whereas the Zulu 2 automatically switches itself on. There have been a couple of times when I have been flying and think to myself that this doesn't sound right. Then when I look down I see that the Bose is off. That's when you really notice how good the ANR is. But even so, if my battery failed in flight I would quite happily complete by journey on passive only. I don't feel I am beyond safe limits for hearing, certainly not in those aircrafts anyway. I do feel I am more at risk from hearing damage just by walking down the streets where I work with lots of road drilling, cars and other loud noises.