Time is a relative thing in a Virtual Machine, and vCloud Director doesn't care about this. Turbo Boost isnt something exposed to vCloud Director itself. When Resource Pools are created, vCenter states the amount of available resources. vCloud Director inheritly uses that.
So to turn the question around, how does vcenter handle this? My guess, is that vCenter bases the amount of available CPU resource on the 2.9 clock speed, not the 3.8 (since that's an overclock, and shouldn't be sustained for long periods of time).