I don't know for your upnp problem but maybe this can help you for making working your XBOX live! ;)
With the specified settings in my post you should be able to clean the MTU problem. What's the link between ports and MTU ?
Well, with my tests, I noticed that the XBOX try to check your MTU by joining a M$ server on the ports 3024 TCP/UDP or 88 UDP.
But, first of all, the request seems to query a DNS server, and then the MTU-Check server through ports 3024 or 88. Don't know.
So, if your DNS test pass, your MTU-Check depend on your NAT settings.
And sometimes your will run 2 times the test and see : NAT STRICT, and now NAT OPEN. :o
I all case, your NAT is OPEN :)
I think its just a bug of NAT detection from your XBOX because its use a remote server ;)
To conclude MTU problem => NAT problem :P