I was looking at MediaWiki, and came across this note about rsync being
fairly ineffective (because of the compression).
http://en.wikipedia.org/wiki/Wikipedia:Database_download#Rsync
So, just in case you weren't aware of it, there is (at least) one
compression program that doesn't have this drawback. There is a gzip patch
implementing a special --rsync (or, in older versions of the patch,
--rsyncable) mode in which it restarts its dictionaries according to the
behavior of a running checksum. This does reduce the compression ratio
slightly (generally somewhere near 2%), but it also results in unchanged
sections being unchanged after compression as well, which means rsync can
skip them effectively. The resulting files are fully compatible with
existing decompressors. Debian's gzip includes this patch (as do many other
distros, I think) and it is planned for inclusion upstream soon.
I don't know how badly gzip fares vs bzip2 on your content, but if it's not
a landslide having rsync work well might be worthwhile...