I doubt that you can reduce the disk IO by that, as the mk_apt script is already very minimal.
The crucial point is the first command, apt-get update. When you set the DO_UPDATE variable from yes to no, this step is skipped in the plugin. However, you need to update the packages lists from time to time, e.g. in /etc/cron.daily.
You can see the required format by simply executing the mk_apt script manually.