I'd love to have at hand some definitive research on this, particularly for participants who have attended my workshops, but the real answer is, "We don't know". And, we're not likely to know via research, because as far as I know, nobody is doing this kind of research. Violence from customers doesn't occur often enough (and isn't reported enough), to determine whether using the verbal defusing skills in terms of doing proper research design.
The logic though, is sound. With situations where a customer commits physical violence, it doesn't "come out of nowhere". There is usually a set of interactions and conversations that occur BEFORE the escalation. It stands to reason that getting better at defusing angry customers will reduce the risks of physical violence, at least from the angry customers.
Physical violence from other sources -- robberies, spousal issues, that occur in the workplace aren't likely to be affected by employees learning verbal defusing techniques.
If We Expand The Meaning of Workplace Violence...
...then things change. Over the last decade or so, the definition of workplace violence has broadened, to include just the kinds of behavior that employees face on a much more regular basis. For example, threats to do harm, verbal harrassment, and other hostile customer behaviors are now included as workplace violence.
I'm confident that using defusing skills will reduce the intensity of length of this type of non-physical violence from customers. Again, we don't have hard evidence -- no research. I wish we did.