Definitely need to check out any additional power/frequency settings that may be present in the BIOS/EFI. Also, once setting the hypervisor to Max Performance, you do need to reboot.
Some other things to consider and look into. Do you have CPU/Memory HotAdd enabled? Do you have 1 socket per core configured? How many vCPU's to you have configured to the VM? Do you have CPU Ready or CoStop on the host?
There's a number of reasons why a vCPU may not have access to the full frequency of a pCPU, and in most cases this is just fine due to the abstraction and how things work in the hypervisor.