have you tried 'muting' the communicator, or un-pluggin it altogether for the match...the talking over the system will use up bandwidth...
i was playing someone who was getting lag, so i suggested we mute, then we just 'un-muted' at half time (or after a goal/shot to say 'well done/unlucky') and he said it made an improvement.
i can't see how stadium settings would effect lag cos i wouldn't think even konami would be stupid enough to send crowd information over the link...would it really matter if the crowd weren't waving flags in exactly the same place and in the same direction on both machines? nope. i suppose it may help the xbox to run the local game better, but it should be able to cope.