On this subject i would like to state several reasons why i think ANY WW2 design would be completely outclassed and obsolete in a modern submarine battle:

1.Noise: the XXI was designed before there was much appreciation of the importance of radiated noise.All modern subs have all there noise generating equipment raft mounted to reduce to a minimum noise that a passive sonar system could detect.To retrofit this to a type XXI could be difficult if not impossible.Also the propellers of a Type XXI would generate a Huge ammount of noise (even if all other equipment in the hull was raft mounted).The reason for this is sometheing called Blade Rate.Basically this means that as the blades of the propellor rotate they pass through disturbed water from the flow of water over the hull af the submarine.As the blade enters this disturbed water it generates a 'thump' at low frequency which can be heard (this blade rate was the a major advantage the west had over the soviets who did not realise it until the mid 80's).the blade rate problem is the main reason why all mpdern submarines have curved propellor blades,the curved blades enter the region of disturbed water more gradulally and do not produce the 'thump'.

2.Hull form.The old WW2 hull form is hydronamically unstable underwater (WW2 subs were designed to opperate on the surface most of the time).Even the XXI did not have a lot of research on its hydronamics (post was American research showed that the XXI was unstable at high speeds).