Check out this old ESX3.5 article about performance differences between vRDM, pRDM and VMDK. You can see that even in these good old ESX3.5 days there was no significant performance gap. As actual VMware Volumes do not have the 2TB limitation anymore, there is no real blocker to use VMDK (and vRDM).
If you use vRDM don´t forget to reserve some space next to the vmx file for snapshots (e.g. Backup-Snapshot-helper) :
http://www.vmware.com/files/pdf/vmfs_rdm_perf.pdf