p2p streaming a la sequential bittorrent may reduce the data flowing through particular peering links but it will be offset by increases either at other peering links or internally. nothing would complicate or prevent that kind of connection from being throttled just like they are doing now.
however, problems come with p2p streaming. you have to have a pool of reliable peers who are constantly on and willing to store and serve your content. this works with bittorrent because your peer network is global and a culture promoting such infrastructure has developed over years, but in a netflix situation trying to avoid congesting ISP peering links, all your peers have to be local and it has to develop overnight. if customers are drafted as peers, they may not appreciate increased usage of their resources, especially when they aren't even using the product. also consider the unreliability of bittorrent: if a seed drops out of the swarm, it's fine as long as the data is available, because your download does not have to be realtime. if you lose a server during a stream, either the other servers will have to increase effort or your quality will simply drop.
they also may be prevented from doing p2p arrangements due to the way content is licensed. they may be able to get around such a limitation by encrypting content, but this might increase playback costs. if they're lucky the already-existing drm would cover it and they can just store it like that, but i don't know exactly how their drm works so that may not be feasible.