Surely your connection can't be the problem, I would like to have a download speed like yours... If GFWL recognizes an Open NAT, I would say that you've fixed your problem, sadly the fact that you can't join bigger lobbies is probably due to a low amount of people playing the game. Since 220.127.116.11 came out, I've never found a lobby with more than 7 players. Do you have this sort of problmes with other games too?
I don't play anything else so I wouldn't know, I do see the bigger lobbies but it puts me into my own lobby every time, I just tried joining a 5 player lobby and it happened, then the same thing happened in a f*cking 3 player lobby so it's definitely something on my end. And the fact that I joined just fine with my older ISP just confirms it.
Also, something weird: I have all ports forwarded but UPnP lists port 3074, but it's already forwarded so I have no idea why UPnP would forward it again.
EDIT: I disabled UPnP and GFWL read Moderate suddenly, so my portforward settings are definitely not working. This is my portforward setup:
UPnP seems to be forwarding the exact same IP that has every port forwarded