It's difficult to determine whether the gap would be bigger if we had better data. One can assume that it probably would be bigger, but it's very difficult to say that with a high level of certainty. It's not known for sure, but my bet would be that it's probably higher.
As to whether it's in the unknown unknowns or the known unknowns, I think that probably falls into the known unknowns because we know there's a problem in that area. We just don't know exactly how big an issue it is. For that reason, it's not in the unknown unknowns, to paraphrase Donald Rumsfeld.