"I owned a share in an L13 many years ago. Fine for local soaring and
no doubt good for basic training, but its performance fell way behind
that of an ASK13, and possibly even the ASK7"

Huh? The book polars for both the ASK-13 and L-13 are nearly
identical. Does that mean that one of them is incorrect? Given that
the Johnson tests of the L-23 and L-33 were very close to the book
polars I would think that the L-13 factory data would be fairly
accurate too. Does that mean that the Schleicher book polar
significantly understates the performance of the ASK-13?

I also note that the DAeC handicap used by the OLC is 79 for the
ASK-13 and 78 for the L-13, short wing L-23 and Ka-7.