With all the "rah-rah" about virtualization and vt support and all, everyone forgot about the I/O.
Your virtual guest is just disk file - probably compressed, and probably copy on write. And the VMWare server is just a process as far the operating system is concerned. So, even if the VMWare is doing direct I/O, there's a fair bit of gumph in the way. Running (I/O) performance tests from the guest is unlikely to be overly beneficial. No harm in trying.