We semi-regularly return to the topic of "fighter" submarines at work as a fun little exercise, either operating on their own or as squadron leaders for a pack of UUV. Since they have much smaller hull diameters the hulls can be disproportionately thick and strong compared to their motherships. Also, with shorter endurance more mass can be put into structure as there are less systems. However the cost of depth for such a boat would be in any water-facing systems. Cooling circuits, sensors etc will all be exposed to the external pressure and these will *also* need to be stronger, which can get really expensive (and risky). This is why Deep Flight uses wings as they can't fail like a pressure-facing deballasting system and don't have some of the... control issues that ascent by blowing one's tanks does.
Hydrodynamic lift is really important for conventional submarines - there are reasons why one would want to avoid using ones ballast tanks and putting a small trim on the boat will generate lift easily (plus the effect of the thrust direction). The trade-off is that the wings add a lot of skin friction, which increases drag - this is a bit of a killer for submarines because of the energy storage problem. (plus drag => noise as it means there's more energy being put into the water one way or another).
RP1