I have a 6 disk RAID6 array on my Adaptec 6805 with the AFM600 flash module for cache protection. All the HDDs' firmware have been upgraded and the RAID card is also running the latest firmware and drivers on ESXi 5.1.
On a native Windows installation I get over 750MB/s sustained speeds. Inside a VM I only get 250MB/s in both a VMDK as well as RDM mode. In fact, even when I unplug a disk and degrade the array, there is no reduction in performance from the 250MB/s.