In this article it is written that the 44 knot setting overloaded the engine:
http://www.uboataces.com/weapon-torpedo.shtml
Like Hitman says, the fastest setting was not used until 1942.
Other sources claim it was never used on u-boats.
In order to simulate the overloading, I use SH3 Commander to randomly change the range of the G7A torpedo at the fastest setting. As a consequence, I hardly use the fastest setting because it is too risky.
Cheers, LGN1