there has always been an endless debate about RDM/VMDK performance comparisons, but since vSphere 5.0 there is really no performance difference between them. The choice for RDM only relates to design consideration and specific needs like Microsoft Clustering, otherwise there is really no more excuse to not use VMDK.
Also, you can convert pRDM to vRDM with only a quick shutdown of the VM, and then using storage vmotion convert vRDM to VMDK, so the effort is low...
EMEA Cloud Architect @ Veeam Software
Veeam VMCE #1