Gladly, I was able to devote some time to update yum fast downloader plugin to work much better with recent yum versions in Fedora 19/20. The plugin effectively disabled delta rpm support since the integration of presto into yum. To fix the issue, I reworked the plugin using the new download framework of yum/urlgrabber. The result is yum-fast-downloader plugin for Fedora 20, which not only fixes the problem but also brings better integration. As a result, the plugin is now responsible for almost all downloads including downloading drpm packages.
Besides, it is now possible to specify arbitrary command line arguments (e.g. -q) for aria2c in the plugin’s configuration file.