Fastest way to move multiple terabytes of information

I do not know much about cPanel, but I know how to transfer a database very fast to another server - if you have access to ssh.

use mysqldump with the proper parameters and chain it with ssh. so the database is imported while the sorce database still exports. no temporary files are used (except mysql internally ;))

sourceserver# mysqldump --user=user1 --all-databases | ssh targethost 'mysql --user=user2'

if you authenticate to the source server with your private key and ssh-agent, you could use the -A option of ssh to connect. So you do not need to care about authorization on target side. But keep in mind:

         Agent forwarding should be enabled with caution.  Users with the
         ability to bypass file permissions on the remote host (for the
         agent's Unix-domain socket) can access the local agent through
         the forwarded connection.  An attacker cannot obtain key material
         from the agent, however they can perform operations on the keys
         that enable them to authenticate using the identities loaded into
         the agent.

(source: man 1 ssh)

hope this helps a bit

Is rsync smart/efficient at detecting which files to copy/delete?

Rsync is extremely efficient at detecting and updating files. Depending on how your files change, you might find a smaller number of large files are far easier to sync then lots of small files. Depending on what options you choose, on each run it is going to stat() every file on both sides, and then transfer the changes if the files are different. If only a small number of your files are changing, then this step to look for changed files can quite expensive. A lot of factors come into play about how long rsync takes. If you are serious about trying this you should do a lot of testing on real data to see how things work.

If the master crashes and a slave takes over for an hour (for example), is making the master up-to-date again as simple as running rsync the other way round (slave to master)?

Should be.

Is there any possibility of implementing multi-master systems with rsync?

Unison, which uses the rsync libraries allows a bi-directional sync. It should permit updates on either side. With the correct options it can identify conflicts and save backups of any files where a change was made on both ends.

Without knowing more about the specifics I can't tell you with any confidence this is the way to go. You may need to look at DRBD, or some other clustered device/filesystem approach which will sync things at a lower level.