Install McAfee GroupShield on Exchange 2007

We found out our current McAfee licensing includes the use of GroupShield for scanning on our Exchange 2007 server.  We currently pay an IT vendor to do AV scanning on Exchange for us but they have installed an AVG product.  We do have limited internal IT resources but we are quite successfully performing all of our AV-AS scanning on a 100 user/12 server network with the McAfee products and managing through their ePO console.  We also currently run all mail through Postini first and plan to continue to do so.  I am considering cancelling that Exchange scanning monthly service and installing the GroupShield but we don't have much experience doing anything on Exchange servers.  I know we can utilize the McAfee support to assist, but was  wondering if anybody in the forum can share what they are doing with McAfee and if you are happy with it's performance or if you have any installation advice before we make a decision.  It could save us a little over $400 per month if we make this change, so it's appealing from a cost savings perspective.  One of my primary concerns is that the performance of the email server not be impacted to a noticeable degree.