Regarding the AI in terms of surface detection. Im working on that now. THe settings i have for AI visuals worked very well in SH4 1.1,, but for some reason not very well in 1.2. Only reason i can come up with, is something else has changed internally to the game.
As for the AI difficulty, i have removed probably 98% of all elite rated escorts (crew rating 4). Overall though i havent had much of a problem with it. Following the guidelines of sorts here
http://www.subsim.com/radioroom/showthread.php?t=104377 and by pushing the boat as deep as it can saftely go before it cracks like an eggshell, the game to me becomes rather easy. As always though i will keep an eye on it. In this last revision i upped the depth charge explosion radius a couple meters because the DC's seemed too inneffectual.
As for historical difficulty, i honestly am not going to delve in to that very far. Ive read reports of depth chargings ranging anywhere from a couple hours to 14 hours to one boat considering surfacing because its battery was nearly totally drained. So opinions on this will vary to a degree. So personally i refuse to get into arguments about whats historical and whats not because ive been in those debates before, and their tantamount to a blind dog chasing its tail.