Hello wp mirror dev,
If you are not already downloading media tarball bundles for the initial mirror setup, you should be. See
http://meta.wikimedia.org/wiki/Mirroring_Wikimedia_project_XML_dumps#Current...
for these, available once a month and I hope to make incrementals regularly available mid-month as well.
Additionally, you can use the 'multistream bz2' files with their indexes to manage concurrency, rather than needing to write out a pile of separate xml files.
Lastly, if you are using importDump.php, I strongly recomment you use mwdumper or mwimport to create an sql file which you can feed to MySQL, and then stuff in the various link tables as well. This will cut down the setup time immensely.
Ariel