I've only ever used the 4" deck gun myself since I tend to start my campaigns in 1941 and I haven't ever made it long enough to get anything bigger.
Do you have a 'work in progress' version of your mod downloadable someplace that works under 1.3? 'cause if so I'd sure like to try it out! Oh also have you or do you plan to mod anything beyond making it non-gyro-stabilized -- like changing the ROF or damage levels? I imagine the stock gun w/ it's ability to sink "huge european liners" w/ 10-20 shots would still be overpowered even without the stablization.
I'll go do a search 'round the forums & see if I can track down your mod to DL.
nomad_delta