It was too late at this point by June 1943, even if the Walthers HOOH Boats worked (that they wouldn't, given the postwar RN experiments with Peroxide power).
KM has a window of time for Subs to work to knock the UK out of the War, and that ends once the USN shows up in force and the air coverage gap is closed, it's over. KM is just sending crews out to their deaths, with little to show for it.
Marginally better subs just isn't enough to stop the convoys across the Atlantic.