lets first understand why escort AA boat is favoured with fast reload guns.
well, escort aims bad, that's why you need fast reload to counter that.
what's the point of having escort AA ship, which will miraculously start to engage at 25 angle, hitting your own team planes (both IJN and MN have such long range guns, though they deal very small dammage to the planes) and then stop AAing while having planes over him, due to gun's limited max angle... ?
so you want guns with fast reload and high max angle.
but words are words, best is to figure it out yourself what you need. So try some different nations AA platforms with different gun setups as escorts.