I have upgraded my PC from a beefy 4790K (4 Ghz 4 core) machine to a crazy dual CPU Xeon E5-2670 for a total of 16 core / 32 thread @ 2.6 GHz. So from fast 4 cores to slower 16 cores.
I did not do a fromal benchmark but here are my findings for those considering such an upgrade:
Rendering is faster... but not much faster. I tried Main Concept, Sony AVC and (my favorite choice) libav H264.
Each core runs at a lower speed and not all cores are utilised and even when utilised, cores seem only to run at half power:
So at best I get about 16 to 20 threads working for a total processors efficiency of 40-45%. The GPUs are barely used during rendering so not the bottleneck. 64 GB EEC memory, 15% utilisation, also no bottleneck.
I have the feeling I gained maybe 30-50% rendering time.
But editing in Vegas Pro feels a bit slower. There, the 4 Ghz really helps with editing speed vs the native 2.6 GHz. I do get immediate full preview like before in best, but it just does not feel as snappy.
So in conclusion the upgrade is not a clear win and below my expectations. Niether the rendering nor the editing are designed to make full usage of so many (slower) cores.
If I were to build another editing maching I would probably go with a sensibly priced 6 core 4930K, 5820K or 6800K.