Bittorrent has never worked well with large numbers of small files, the overhead outweighs the benefit. Addons are hardly large alone, the issue is only when a large number of people download a large number of them.
Indeed. I've discussed the idea of using p2p to help distribute mods. Hell we've even talked to bit torrent's designers. End conclusion: Torrent is good because the files are too small. p2p in general is possible but would require something custom.