For your switches, make sure that igmp snooping is enabled. Also remember that the slowest device in your multicast group sets the pace of the multicast stream. If you are imaging 9 i7s with nvme drives and 1 pentium 4 with a sata hdd the multicast group will operate at the speed the pentium 4 system can digest the data. Or if you have 9 machines on a 1GbE network and one machine on a 100Mb/s link, the speed of the multicast stream will be restricted to the 100Mb/s link.
60mb/s is not a value that FOG would offer. If that is measured bandwidth speed, ok. But FOG’s partclone measures in Dataflow over a minute. So what speed does partclone say during the multicast stream.
I might try a portion of what you proposed for a test, but instead of disconnecting that imaging switch from the rest of the network, leave it connected. The switch should keep all multicast traffic on that switch as long as there are no subscribers anywhere else the network.
A on a healthy 1GbE network you should get about 6.1GB/m transfer rates. For a multicast on that same network you should get between 4.0-5.5GB/min. On my network I routinely get 13GB/min+ unicast image to modern hardware from a virtual machine FOG server. Just to set some expectations.