I agree about weights and guns. I think it was a good choice the more comon 5".
Sorry when i refered 4cm "what you loose" was the industrial retooling necessary for a sizable production. Let's say some resources would have to be taken from somewhere.
When a ships goes to a yard for such long time(months) it looses always a big part of the crew, a new captain etc, that needs a training period. Anyway an even better example: Izuzu convertion to an AA cruiser was 4 months
http://www.combinedfleet.com/isuzu_t.htm. But the ship was repaired and overhauled in drydock 4-5 months before. So from Jan44 to Sep44 it was in shipyard.
I know it is pain. the conversions of 2 weeks ended. For example when reading TROMs of British destroyers going to dock and putting some radar , adding some 20mm oerlikon is usually 2 months or even more(includes obviously new painting, hull cleaned, engines rebuild etc...), not all conversions are like that, but most of the time they take the chance to do a ship overhaul.