Due to unforseen circumstances we had to cancel our conversion, but we ran a test instead.
We used our SAN to create a copy of the 9 TB LUN/RDM.
We attached that copy to a test VM in Vcenter v5.5
We used a cold storage vmotion to convert the RDM to a thin-provisioned VMDK.
The data traversed a 10GB fibre link, but our old SAN is limited to 4 GB fibre channel (due to old Cisco switch), whilst new SAN has 10 GB links. Basically we're limited to a 4 GB fibre channel speed.
The RDM only had 3.13 TB of used space (just an FYI).
The conversion took 17 hours.
In case anyone is wondering.