It's likely going to take a very long time to get any kind of worthwhile data back on that. Not only does it have to work to begin with, but the person it works for will have to then make it known in the forums, or contact someone that likes keeping up with data.
Eventually, maybe you and some friends will have the missions enough times, that youll be able to get an idea of how often it would occur. I dont suspect it's going to be a very high chance of success.