Koji is not a mock replacement and afaik koji still uses mock for the actual build process, and will continue to do so.
Koji only makes sense if you have some more machines to setup and a larger group to work with. Otherwise mock is much faster to use and tweak.
btw: It is great to have to "how to build CentOS" wiki more active and actually collecting additional tools to use.
regards,
Florian La Roche