I'd wager it's largely disruptive and dangerous in a highly localized way due to the small percentage of folks doing it. Doesn't make it an acceptable practice though. One person "rolling coal" can temporarily blind 3 or 4 cars back and several across depending on wind conditions, etc.
In terms of NOX it can be a factor of 100. If 1% drive without cats they produce half the NOX emissions. In reality it is probably less since there are other old cars as well that have higher emissions
