I understand your pains BigAnvil. I will try to give my thoughts on your suggested config.
My personal thoughts are that you should go with all the extra disk controllers. Just stick with what's in the box at the outset (and monitor throughput of course).
You have mentioned that you have 500 mailboxes. I don't know what sort of company you work for. Are they 500 information workers - ie heavy users of Outlook? Office based? Do you already have mailbox quotas in place? And Send/Receive message size limits? How long do you intend to keep the data for? I am thinking you can do some quick and dirty multiplication of the mailbox sizes, retention, and so on to try to work out the possibility of filling the 2.5Tb of storage.
You can always move the data around after the fact, but it becomes a little painful... but probably not too bad in a medium / small environment as you have outlined here.
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