Just wanted to point out that there is nothing as quiet as a diesel electric doing nothing. Even an old dog as this. It simply makes no noise. As such a bottomed out VII could have done well in some places, like the Gibraltar straight. With homing or pattern running torpedos? Come to daddy!

Secound thing is the XXI was the father of allied subs after WW2. In just about all areas but radar it was superior to allied subs untill nucelar power came.
If I remeber correctly XXI could remain at silent running 5 knots for 340 miles without recharging.
As for the boat being primarly a surface vessel? It had no deckgun and was equipped with schorkel, and had greatly reduced charging time to cut down on time spent at the mercy of radar at PD.