Looks like I finally got it. I will document. The iMacs are getting a solid 37.4MB feed from the server. Here is the network activity graph from the Snow Leopard server I use for DeployStudio.
The spike is the check initially when I started 3-4 iMacs via DeployStudio on individual streams. The server bonded nics spiked on the way to 160MB. When I got multicast going, the server started dishing out a consistent 40MB but each of the 4 iMacs in the test were getting the same speed.