The ability to read and understand any (reasonably well written) code in that language is the most important criterion to measure mastership.

The ability to write code to solve any problem doesn't say much; just consider a C programmer who claims the ability to write C++. Since C is more-or-less a subset of C++, the C programmer can solve any problem by writing an appropriate C program and using the C++-compiler to compile it, so technically it's C++. Likewise, many programer learn just enough of a new language so they can apply the old idioms. *You can write FORTRAN in any language*.

By the same argument, LOC/hour says nothing. A programmer who writes Perl programs that look like C programs probably generates more LOC/hour than a Perl programmer who write idiomatic Perl code.