On our test Plesk server (Not Central Mail, thats a later discussion)
We used "DD" to duplicate our active drive then, we went from 7.5.4 to 8.0, reboot, install extra packages, 8.0 to 8.1, reboot, install extra packages, 8.1 to 8.2 reboot, install extra packages. We just did the reboots arbitrarily, we wanted to make sure the upgrade truly worked.
Just remember with the "Updater" you have to wait a long time, do not believe the email that says the update is finished. Open "TOP" and watch and then watch some more. If you do not wait the whole process will be for not.
The centralized mail server we just cloned the drive and went from 7.5.4 to 8.2 and it worked, not the first time though, we didn't wait before we wanted to play with it and reboot, it did however finish even with the abrupt reboot, but it took a long time.
We do not have slow servers or drives and we have a hardware mirror so we can pull drives every 2 weeks, We are guessing the slowness was the DB table re-configuration.