Storage Sizing: IOPS per mailbox

[ram]

When sizing a storage device for a large cyrus server, the typical
question asked by storage vendors is what is the IOPS required per
mailbox 
M$$ Exchange has this concept of IOPS. and they suggest 1.5 IOPS per
mailbox ( heavy users ) 

If I use postfix and cyrus , on my imap server ( pure IMAP server .. All
spam filtering , outgoing mails , authentication etc happens on
different servers )


If the storage is used only for imap storage , what is the typical
"IOPS" requirement per user
We will probably assume 30-50 mails a day of average 100k , and an email
client checking for new mail every 5minutes