i have just basically done this, rolled out 1000 phones globally connected back to a cluster in UK. All the phones are 7965 and it was taking an age to upgrade the phones sometimes upto 45 minutes even though at most sites we had upto 10 mbps MPLS. I used 2 methods, first i build a VM on my laptop and added the required version of firmware we had on the cluster, worked a treat and the phone booted fine, obviously just change the option 150 dhcp information while you upgrade. The other way is the new peer to peer upgrade method you set this on a local phone. hope this helps.