1 British 3in/60 Pdr rocket does 187 HE damage. 1 AP round from %75 of the 75 and 76mm AP rounds do 78 lbs damage. Once could estimate that the 57mm AP round would do LESS than 78 lbs of damage vs an OBJ. Granted, there is no real way of measuring which one would be more effective vs a ship, but may I suggest trying a few runs against a fleet using the Me410 and the 50mm HE that does 50 lbs of damage and see how that goes?
The Tse Tse would be fun to play with, but again I'm not sure what it would be good at. Maybe busting gv's, but that is anyone's guess.
The Mollins is supposed to have a significantly better armor penetration than the German 50mm. They sunk ships with that cannon. It was also used in air to air a few times, including in dogfights against 109s and 190s, but if I remember correctly its only flying victim was a Ju88. The rockets were indeed found to be much more effective and the numerous MK VI could carry them.
In AH the XVIII will be used against GV and as a de-acker as it will be able to snipe the ack position from a much greater distance than the 20mm. The added armor may help a little against the auto acks and GV fire up to 20mm. I doubt it will survive an Ostie 37mm hit.
Personally, I'd love to see it, as well as the Mossie XXX model. The latter will be inferior to the Mossie VI right on the deck, but I suspect that already at 10k it will start to be superior. It is a much better bomber hunter than the VI.