If it is a bandwidth issue then GRAW is extreamly heavy on net code 8.5k per client going by your 1024k upstream with 12 players, but to me that does'nt add up because if I host with 256k up I can host 8 people. And someone who had slightly more upstream than me 384k could only manage the same number of connections.
There's 2 ports that the xbox uses 88 and 3072, most people know to have these open otherwise it causes NAT issues, we were trying with DMZ enabled. which wil prevent NAT issues locally.
At the moment we can only speculate I suppose whats causing the issue, some have mentioned the load on the Microsoft 360 live patch server was too heavy last night, The patch server apparently diverts traffic to the hoster, but again this could be just speculation.
I would like to see someone from the GRAW dev team confirming the requirements to host and join or if there is an issue they are aware of.
well would be nice