is_ssl() may not reconise the alternate port in some configurations
not all ssl links are generated through that function (unfortunately), there are a number of cases of code such as: wp_redirect('https://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI']); (wp-login.php)
This could also be achieved using a filter (other than the above case where url's aren't generated with filtered functions)
CUSTOM_PORT would be better off named SSL_PORT or similar.
(Pulling out of the Security component since it's not a security issue in WordPress)