Virtualized Servers for Performance Testing

We have to do benchmark testing as well as stress testing. Already high performance system was purchased and virtual machines are created for performance testing setup. I can see few limitations for this approach, since production boxes all are in physical servers. Earlier I've used virtual machines for automation regression execution.

Issues/Limitations of Virtualized Servers

1. if (production environment != load test environment) then result could be wrong
2. Increasing the risk of deployment failure

Did a google search and found following links, which give all the details, including Vmware claims.

Virtualization: Performance testing

Measuring the Performance Impact of Virtualizing a Web Application Server

Response to 'Load Testing a Virtual Web Application'

Virtualization performance testing tips


Anybody have experienced load testing on Virtualized servers? Please share your experience...