I'm really interested because - All I've seen over the years is that a community has toxic people in them and it's either because of their actions or (funnily enough) because of their initiative that a CoC gets drafted and put in place. Afterwards, these toxic people now see themselves in charge of upholding the CoC and you get a neat little mall cop situation.
It depends on the CoC, of course, but in my experience the actual details only change the rate of decline in community quality. If it does end up getting better, it's often in spite of the CoC, not because of it.
Inevitably, people who aren't as anal about the rules find themselves pushed out of the community by sheer frustration over being policed by the mall cops. More often than not, the people who are pushed out happen to be the more productive members of the community (in other areas than policing).
I had an experience where I was policed for using the "highly offensive dictionary ``complete cock´´". Not towards a person, just as a general expression. I was so astonished, I had to make a screenshot. It marked the beginning of the end of my participation in that particular forum.
A year later, another developer was pushed out of the community for using the word "pussy". As in: "I decided I will finish this code tonight instead of being a pussy." Fun times.