I witnessed a DD get hit by a mk.27 I had fired once while I was being chased by 3 DDs the thing ran for some time and I was tracking it on the sound gear with great disappointment as it was way off track from any of the DDs so I stopped tracking it.
Suddenly after a very close and nasty DC run on me a minute or so later I hear boom! "torpedo hit sir shes going down!" My guess is that the loud noise of the depth charges going off must have attracted the sound seeker inside the Mk.27 and turned around and pegged onto one of the DDs killing it.
What makes the cutie so weak is the low speed sometimes they get a good track but just cant keep up so they work best against a searching ASW vessel who is varying his speed to attract the cutie but he is turning and slowing down enough that the cutie can hit.
Funny as the USS Barb did get a medium sized merchant in WWII with a Mk.27 though they had fired it a almost 90 ft and it came up under and hit the bottom of the ships hull other wise I doubt a cutie alone would be able to sink anything larger than a DD.