I did some tests with rsync that showed some good improvements rsyncing tar files rather than .bz2 or the whole portage tree. Read it at http://forums.gentoo.org/viewtopic.php?t=10108 Maybe its worth thinking about for the next portage. I'm also interested if any of the portage developers have tried something like this or experimented with different rsync blocksizes.
http://bugs.gentoo.org/show_bug.cgi?id=5982 also deals with improving rsync
err, i meant Bug 5981, sorry
This isn't too feasible. (then we'd need a central mega-rsync distfile repo). Closing this bug.