Going to HTTP as transport is a very bad idea because HTTP is a very inefficient protocol. There are way too many overheads for HTTP to handle that kind of transport. Better use would be FTP (or other derivative versions of file transfer protocol that's meant primarily for transfering of files with much lower overhead) if they want to go with a centralized method and BitTorrent if they want to go the P2P method.

[My gut tells me that SE is probably using some form of derivative of rsync currently and is the cause of the bottleneck]