Sorry for the lack of posts recently.
At this client, we are currently in the middle of Empirix Performance Testing (http://www.empirix.com/). Interesting stuff and progress to date!
We have two G5 Hammer Load Generators installed at each of our Contact Centre sites as well as a Genesys VAS (Virtual Agent Simulator) at each site. Calls are being injected directly into the Avaya switch via SES and a SIP trunk.
When I get a bit of free time I will update this post with some of my findings.