Dovecot proxy

[Gandalf Corvotempesta]

Hi to all
I have some *production* pop3/inappropriate server that i would like to
move under a proxy

Some questions:
1. Keeping the same original hostname on the proxy (in example
mail.mydomain.tld)
and changing the hostname on the imap server, makes some troubles like MUA
redownloading all the messages?
Is dovecot (running on the imap server) happy seeing the hostname change?
What about maildirs, where the hostname is wrote on the mail file?

2. Dovecot proxy will proxy the whole pop3/imap traffic or only the login
auth?
I don't want to expose the mailservers to internet, all imap session must
be proxied through the proxy.
this because I'll use local IPs on each mail server.

3. Performance for the proxy server?
The same as the mailserver or higher due to the missing email computation?
In example,  the proxy doesn't have to access disks or emails data but has
only
to transmit what the mailserver osd saying

4. Like question 3, any real users for the proxy?
I would like to know some info about hardware and userbase (in example:
dual quad xeon 5600, 32Gb ram, 10.000 concurrent sessions)
In my case I'm planning for about 100 active sessions. Can i use a small
EC2 instance?