I love what you're doing to try to make the D/C experience more realistic, BUT, I hate to say it but this is a pretty meaningless poll/question the way it is posed in a relative vacuum.
If (and this is an assumption I acknowledge is true for me but not necessarily true for everybody) the "best" realism is one which achieves a realistic end result - in this case a realistic chance of escaping with damage vs being destroyed - then you can't really answer this question about what an essentially direct hit would do to your sub unless you can also answer/address the question of whether the percentage chance of these direct hits is reasonably accurate/realistic in the game. Obviously, if it is, then you would want the results of those direct hits to also be reasonably accurate/realistic. BUT, if for some reason the AI escorts typically get a much higher percentage of such direct hits than would be historically accurate, then for sure you would not want to make the effects of such direct hits be too lethal (even if they would be IRL) because then you would have distorted the lethality outcome too much to the deadly side.
From the comment below ("Watching a zillion DC attacks with external cam on, I was stunned at how many DCs it took to kill me where they were touching or even inside the sub.", it's hard to tell for sure what was happening on a percentage basis, but it sounds like maybe there were a large number of direct hits. If there weren't all that many US subs lost due to D/C attacks, and the game is throwing off a lot of direct hits, then it doesn't sound like a good idea to make direct hits too lethal unless you can also reduce the number of direct hits experienced.
The best result would seem to be some way to increase the number of minor (but significant) damage results, such that D/Cs would be something to fear and could cumulate to deadly damage, but where the player would still have a reasonable chance to at least try to deal with the damage most of the time. After all, if you're not playing with external views on while you're being attacked, then you really don't know or care how close the D/Cs really are (i.e., whether they're 20 feet or 30 feet away) - all you really know is are they close enough to be damaging you or not.
Of course, that assumes that the damage control model in SH4 works or can be made to work like it eventually did in SH3 with LRT. So far, I haven't encountered the right circumstances to test the damage control system meaningfully, so I don't kow whether it works or not, but I hope so - otherwise, you might as well go for the instant screen of death with super lethal D/Cs and spare us the disappointment of dealing with a damage control feature that doesn't work.
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