SO I got home and ran a few tests. With and with out DNS Relay. Both xboxes were OPEN NAT if powering either one ON using Specific Remote IPs.
So I figured all this DNS talk going on, why not re-run Name Bench to see if maybe there are newer custom DNS servers to use. Sure enough, it found a couple new ones. I input them into the manual Internet settings and rebooted the router. DNS Relay is OFF, GF and uPnP ON. Both xboxes are OPEN NAT. Was only able to play with 1 xbox however it was good gaming and ALL green bars in BO2.
None of the new DNS were Google related:
NTT
OpenDNS-3
Speakeasy Seattle US
Will give some time using these new DNS servers a try. This may have been an issue the other night when I saw Moderate NAT on my xbox while the other was OPEN.
I might suggest that drunkJug use NameBench to find some possible newer and faster working DNS servers to use for the 4500 and see if this might help some. You'll need to run Name Bench with a PC directly connected to the 6120. Takes about 5 minutes to complete the test.