I wanted to point out this functionality in case others want to test it:

http://centosnow.blogspot.com/2014/03/createrepo-and-parallel-delta-rpms-on....

For the record, this cuts down the metadata generation to 1/4 of the original time on my test platform.

Other relevant links:

http://lists.baseurl.org/pipermail/yum-devel/2014-February/010580.html

https://github.com/imcleod/createrepo/tree/feature/parallel_deltas_full

http://people.centos.org/hughesjr/createrepo_delta/

Thanks, Johnny Hughes