Is it? Unless you've worked with the backend code yourself I don't know how you'd be able to determine this.
My experience with netcode is that as soon as the netcode gets a bit better the designers use that improvement to add more entities/players. To the player it will continue to feel like the "same old" netcode, but there will be more going on than before.
I haven't run any numbers or anything but it has been my personal experience that 360 games have larger player limits than in the past, more entities, more things going on, etc., than ever before. Without advances in the backend networking I have to wonder how they'd manage that?
Sounds like just because the only "advances" you see are the value adds MS is doing to try to differentiate Gold from Silver you're assuming that they're the only advances.