OWC CPU Upgrade Program for 2013 Mac Pro Now Available
Down the line an upgrade of the CPU in the 2013 Mac Pro is a nice option to have.
But for those who want the fastest now, order the 4-core model (1TB flash drive and D500 or D700 GPUs) and then upgrade the CPU to the 8-core 3.3 GHz. That is what diglloyd.com photography and MPG use. See also 2013 Mac Pro: Choosing the CPU and more.