A problem of these is that just like most other things, it might historically be Ok to make a few fixes and improvements. But to upgrade a Type II sub hull to have the same offensive capacity as a Type XXI submarine? That isn't something really historical.
That problem can be fixed easily by adding some sea & convoy attack to the hull tech (better hull = more space for torps, so it would also work for other sub models).
As for me, not being able to upgrade torps for subs is kinda strange. On the other hands, subs seem to be overpowered anyway. Having electro torp by 1942 and chewing the RN to pieces with subs feels... awkward...