sometimes it works other times it doesn't.
I never heard of a PBY with a 57mm, I think you are thinking of a German plane that had one and was used for anti-shipping.
The problem with a PBY would be it not filling any role in the game that already is not filled. It would not be a formation as I'm almost positive it did not have a bomb sight so cannot be used for level bombing. Also modelling the plane to land on water without damaging it the way that our planes do now would be time consuming. There are very few ships that it would be able to sink anyways. Suplly convoys are rare and It would never get close to a CV. If it did get to the CV it does not carry enough ords to sink it anyways. If you really want a plane that can be launched from ports maybe consider the various float planes that were used by the Americans and Japaneese. Some actually were not half bad planes.
example: A6M2-N
"The seaplane base in Jaluit, for example, is known to have harbored in mid-1943 the Mitsubishi A6M2-N's ("Rufe"), a float plane version of the "zero" fighter"
http://marshall.csu.edu.au/Marshalls/html/WWII/SeaPlaneOps.html is where I found that, might be able to find stats as well.