Doing this causes this error at the end of the operation :(
Incompatible device specified for device ’0′
One solution is to clone the disk using vmkfstools, but after the clone you need to remove the RDM and add the cloned VMDK by adding an existing disk to the VM configuration. My test was with a 40GB disk.
You can time the operation, I used this command
time vmkfstools -i /vmfs/volumes/lun1/test-vm/test-vm_1-rdmp.vmdk /vmfs/volumes/lun1/test-vm/test-vm_2.vmdk -d zeroedthick
Clone: 100% done.
real 7m 54.56s
user 0m 0.63s
sys 0m 0.00s