Nice! The following seems to have slightly better worse-case:

preg_replace_callback(
 '#(%(?:' . substr(chunk_split(bin2hex($reserved), 2, '|'), 0, -1) . '))++#i',
 function($m) {
 return rawurlencode(strtoupper($m[0]));
 },
 $s
);

On the other hand – its slower for common inputs so probably not worth it.

Reserved characters are rare, so probably not.