I have often thought that this tech should still have the discount BUT ALSO allow you to build +2 naval units in that sea zone.
Therefore, if England had an IC in Egypt and Improved Shipyards, then they could build 4 naval vessels in SZ 34. (And in turn, Improved Factories would be limited to only ground/air units.)
I also feel that factories should have double capacity. You may build 10 ground/air units in Germany AND 10 Naval units. (so you have different counters.)
I also think that the destroyer tech should be removed and replaced with AA Guns on Battleships. (Technically you should put them on Aircraft Carriers since they were the ones most heavily garrisoned with AA Guns I am told. But in terms of game play, it is the Battleship that should have them. It encourages their purchase, justifies their cost, and aircraft carriers are already plentiful and purchased during the game.)