The only change it makes to the packets is rewriting the destination port, it's not possible for it to be mangled in a means that does anything to the protocol within (if anything it changes were broken, you would never get a connection via telnet as it would break the most basic of TCP communications). Those aren't exactly easy protocols to troubleshoot via packet capture since they aren't nicely decipherable like HTTP, SMTP, others, but that's worth a shot.