It would be cool to find out just what the real statistics are. I'm pretty sure, though, that Israel classifies this information as a state secret and we may never know in our lifetimes.
The rockets generate more psychological damage then physical. As far as weapons go, they are rather pathetic. All the iron dome really has to do is to make those it protects feel safe. If statistics have the potential of damaging this feeling of safety then you ca be assured that they will be kept secret.
The other purpose of the iron dome is to limit the desire to fire the rockets in the first place. If one thinks their efforts are in vain then they are less likely to follow through. If Israel can convince members of Hamas that their rockets are not working then there will be fewer rockets launched at Israel.