My project is over and I thought I'd post what I found that gave me piece of mind.
I opened a ticket with the Zimbra Engineers and this is what they suggested to load test my proxies to ensure they would work with the amount of load I will be throwing at them.
MStone -
Mstone performance testing tool - The modern version of the application that Netscape Messaging Server shipped with that could load test their product. It's pretty slick with its graphing support.
They also suggested to read ajcody's wiki entry on the subject, which by the way he has many amazing entries in the Zimbra wiki. Be sure to check out what he has to offer.
Ajcody-Testing-Debugging - Zimbra :: Wiki
I found this tool to be very useful and my original question to be answered. Slán.