Ok, well, really, saying high threashold is a little misleading, what I should have said was high gain reduction, as the threshold efect is dependant on input level.
So, if the gain reduction meter is reading high levels ie more than 6db, then you are looking at a heavy setting.
High ratio?
hmm, subjective I suppose, but I would say 6:1 and up is starting to get pretty hard.

No I wouldn`t suggest distortion etc on the master bus, but on a group bus it can be controlled more easily.