This is more an enchantment related on pacman (or better, to libalpm, I'm working on it) but there is a trick to make it work on pacman without modification (see https://bugs.archlinux.org/task/20056?project=3)

the step are:

- run pacman Sup to get the list of all the package that need to be downloaded
download them in the cache folder of pacman (im thinking on parallel with torrent + webseed)
- run pacman -Syu (eventual missing package released during download of the rest of the package may happen)

Stay updated with pacman issue to know when and if i will release the pull request for native parallel download; Im having some issue with the internal CURL, so it could be that i will enable only when using Xfercommand