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Most of the time you should not see any performance loss
Quotation taken from the following document http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1005764#performance
If I add newer hardware into an EVC-enabled cluster with a lower EVC mode, do I lose performance?
All CPU features provided by your host hardware are available to the hypervisor. Optimizations for CPU virtualization such as AMD-V and Intel VT-x or facilities for MMU virtualization such as AMD RVI or Intel EPT support are still used by the hypervisor. Only those CPU instructions that are unique to the new CPU are hidden from virtual machines when the host joins the EVC-enabled cluster. Typically this includes new SIMD instructions, such as the latest SSE additions. It is possible, but unlikely, that an application running in a virtual machine would benefit from these features, and that the application performance would be lower as the result of using an EVC mode that does not include the features. Check with the application vendor to determine which CPU features are used by the applicatin.
I hope this helps