You simply need to copy the current function sanitize_title_with_dashes from https://core.trac.wordpress.org/browser/tags/4.6/src/wp-includes/formatting.php#L1948 and change the following line:
$title = utf8_uri_encode($title, 200);
To this:
$title = utf8_uri_encode($title, 1000);
Minor edit: Always refer to the source for the same version of WordPress you have installed. In most cases, it should be the current version.
The script you are using does not match the current function from formatting.php (compare differences here). Particularly, the following two lines appear after the if ( 'save' == $context ) condition is triggered in the original function:
$title = preg_replace('/&.+?;/', '', $title); // kill entities
$title = str_replace('.', '-', $title);
Example
Below is the same function as sanitize_title_with_dashes renamed as your new function wpse8170_sanitize_title_with_dashes. Only the value in utf8_uri_encode() has been changed:
function wpse8170_sanitize_title_with_dashes( $title, $raw_title="", $context="display" ) {
$title = strip_tags($title);
// Preserve escaped octets.
$title = preg_replace('|%([a-fA-F0-9][a-fA-F0-9])|', '---$1---', $title);
// Remove percent signs that are not part of an octet.
$title = str_replace('%', '', $title);
// Restore octets.
$title = preg_replace('|---([a-fA-F0-9][a-fA-F0-9])---|', '%$1', $title);
if (seems_utf8($title)) {
if (function_exists('mb_strtolower')) {
$title = mb_strtolower($title, 'UTF-8');
}
$title = utf8_uri_encode($title, 1000);
}
$title = strtolower($title);
if ( 'save' == $context ) {
// Convert nbsp, ndash and mdash to hyphens
$title = str_replace( array( '%c2%a0', '%e2%80%93', '%e2%80%94' ), '-', $title );
// Convert nbsp, ndash and mdash HTML entities to hyphens
$title = str_replace( array( ' ', ' ', '–', '–', '—', '—' ), '-', $title );
// Strip these characters entirely
$title = str_replace( array(
// iexcl and iquest
'%c2%a1', '%c2%bf',
// angle quotes
'%c2%ab', '%c2%bb', '%e2%80%b9', '%e2%80%ba',
// curly quotes
'%e2%80%98', '%e2%80%99', '%e2%80%9c', '%e2%80%9d',
'%e2%80%9a', '%e2%80%9b', '%e2%80%9e', '%e2%80%9f',
// copy, reg, deg, hellip and trade
'%c2%a9', '%c2%ae', '%c2%b0', '%e2%80%a6', '%e2%84%a2',
// acute accents
'%c2%b4', '%cb%8a', '%cc%81', '%cd%81',
// grave accent, macron, caron
'%cc%80', '%cc%84', '%cc%8c',
), '', $title );
// Convert times to x
$title = str_replace( '%c3%97', 'x', $title );
}
$title = preg_replace('/&.+?;/', '', $title); // kill entities
$title = str_replace('.', '-', $title);
$title = preg_replace('/[^%a-z0-9 _-]/', '', $title);
$title = preg_replace('/\s+/', '-', $title);
$title = preg_replace('|-+|', '-', $title);
$title = trim($title, '-');
return $title;
}