How can I hide the IP of registered commentators?

Just add a check for the log in status:

add_filter( 'pre_comment_user_ip', 'wpse_53935_no_ip_for_logged_users');

function wpse_53935_no_ip_for_logged_users( $ip )
{
    return is_user_logged_in() ? 0 : $ip;
}