Just as an update. I had initiated a Cisco TAC request and they did ultimately get back to me, first acknowledging that they could replicate the issue in the lab and then a couple of days ago with a beta firmware which has resolved the issue completely. I am now seeing full anticipated download speeds (approx. 930Mbps +/- down from a 1Gig service) with antivirus enabled. So, took a while but it looks like it is now resolved. They say that this fix will be incorporated into the next firmware release. For now, if you are seeing the same issue they are asking you to initiate a TAC request and they will offer up the beta firmware.
Just thought I would pass that on.