Hey Carl, The current firmware for the 5168N is 10.5.4, from our testing we found no issues with port forwarding or using DMZ+ under normal circumstances. Have you been seeing anything odd with these protocols?
In the past there was issues with PPTP VPN Protocol 47. I personally have had issues with a 6to4 tunnel because Protocol 41 has been blocked. I was told someone would look into this and have not heard back. I would really like this fixed but its not easy for me to just test it, as it requires a major configuration change.