The client mail server is back up. The copy took less time than I had anticipated owing to the faster disk subsystem. Mail is now on a RAID10 system with 4 Western Digital 7200RPM Black drives. These drives are each capable of sustained write speeds of around 120MB/s and the machine has adequate bandwidth to fully saturate all four of them continuously. This should make for a much nicer mail experience.
There will be some slowness for a couple of more hours as I move some other hosts. The network interface is 1GB and this tends to saturate it when I do these moves but it will help balance the load system wide to provide the best user experience possible.