Now I am thinking of taking that same thinking and getting rid of the deliver over nfs and deliver directly to the raid server like you said.
We have such a setup. 3 MX servers running postfix and antispam software. 2 clustered (drbd, heartbeat) IMAP/POP3 Servers
The 3 MX server deliver via smtp to the imap cluster and there runs another postfix instance without spamchecks.
Keep in mind your alias setup - our mx servers expand the aliases before they deliver to the backend postfix. Be sure to disable the aliases on the backend mta otherwise you will get your mails twice. normally the backend mta should not send any mail but if there are problems with the quota it should reach the sender, therefore we have enabled the aliases only for outgoing mail. this helps in the special case if an internal users sends to another internal user which is over quota and the senders address is an alias :)