I've seen chain ground a good ways past the witness mark with probably 95% of the cutters still on. That said, it did NOT cut well at all, and I wouldn't recommend it.
You can grind them till the teeth start breaking off and not be in much danger.
A chain seems to me to get faster as it's filed back, to a certain point. After that it starts going down hill pretty quick. So to me it's not worth taking the teeth back that far.
I've noticed (in race chains) that when a tooth breaks off it will stick in the log, taking out several teeth behind it.
Some of my faster chains are the ones at the end of their lives. Assuming correct angles, they still hold an edge properly, clear chips beautifully, and touch up with a file just fine - lots of room to work. Now, they may worry Woodie a bit when they get ground this far back, but even he will agree that they can cut!
My local Stihl dealer must think the witness marks are the same as crosscut marks on a piece of lumber. Not much left after he gets done "grinding" a rocked chain. He did one chain for me. He will never do another.