Rename W3 Total Cache admin menu

You may be able to use the WordPress filter ‘gettext’:

http://codex.wordpress.org/Plugin_API/Filter_Reference/gettext

add_filter( 'gettext', 'rename_menu_item' );
function rename_menu_item( $translated ) 
{
    $translated = str_replace( 'Performance', 'Ytelse', $translated );
    return $translated;
}