Well, I would expect a pleasant synergy for sure. As far as potentiation, it depends on how kratom is metabolized, I guess. I don't think I ever hear about opioids for potentiating benzos, but for potentiating opiods, benzos work amazingly well. Too well, if you're not careful. While the mitragynine in kratom works as an opioid, that doesn't mean it'll for certain be potentiated with benzos and I'm pretty sure it isn't related in structure to other known opioids, so that probably lowers the chances of it sharing the right metabolic pathway. I'm guessing.
EDIT: Never mind, I think I found on another post that it is in fact similar to other opioids with respect to how its broken down. They'll potentiate one another.