after a (kind of) detailed research i find out these registered 20mm attack on ships by Type II's:
U-3 IIA
sept 30 1939 sinks SS.Vendia (danish 1150tn) torp+20mm gun
U-18 IIB
august 30 1943 sinks patrolcraft (soviet 56tn) 20mm
U-19 IIB
march 19 1940 sink SS. Minsk (danish 1229tn) torp+20mm
U-20 IIB
june 24 1944 sink 4 small motorcraft (all soviets, one of them amphibious craft DB26, 15tn each) 20mm+charges (no further details)
U-22 IIB
november 18 1939 sink SS.Parkhill (british 500tn) 20mm
U-23 IIB
october 4 1939 sink SS.Glen Farg (british 876tn) torp + 20mm
august 24 1943 sink minesweeper TSC-578 (soviet 35tn) 20mm
april 5 1944 sink patrolcraft CS1672 (soviet 56tn) 20mm
U-59 IIC
October 28 1940 sinks 2 fishing boats, St.Nidan (british 565tn) and Lynx II (british, 250tn) 20mm+charges (no further details but by the time of sinking seems like they were both stopped, one sinked by charges and the other ultimated by 20mm)
U-57 IIC
higher score for any Type II, 66mil tn (just added the data for fun)
No type IID ever sinked (or damaged) any ship with their 20mm, U-138 IID holds the highest score for the sub-type 48654 tn. They were the last Type II to be retired from any front (except black sea ships) in late 1941 (so up to 1942 it is historically accurated to use a Type IID, too bad 2nd turret cames in 1943).
FINAL NOTE:
the largest ship ever engaged with a 20mm was 1250tn ship, but the few 1000+tn ship engaged were in fact previously torpedoed.
The largest ship ever engaged exclusively by the usage of 20mm was a 500tn.
It was more frecuently used as repelishment gun against patrolcraft.
At the end of the day the usage GWX3 de facto allows from the 20mm seems like more accurated to the historical usage German captains gave to them. after all despite being "capable" of sinking a larger merchant, no captain on his rightfull mind will stand during hours shooting a merchant that has shurely sent help signal to allied ships
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