Optimization in convert_string_datum?

I'm reviewing the strxfrm patch, and while comparing that code to the
code in varstr_cmp (which uses the same UTF8/UTF16 workaround but for
strcoll instead), and I noticed that in varstr_cmp we have an
optimization to use a stack based buffer instead of palloc if the string
is short enough. Is convert_string_datum performance-critical enough to
make it worthwhile to put a similar optimization there?

//Magnus