Currently I'm rocking base level 4 core, 3.7 ghz mac pro 6,1. I tried a 10 core ES processor a while ago, but ran into some issues with the processor lacking h.264 embedded instructions, so I had some crashes when I would push multiple video streams. My ES was E5-2670 2011 V2 ES - 2.3 GHz 10 core. Which I liked a lot, the single core was a little laggy, but overall rendering and virtual machine performance was tip top.
Well theres only a few processor upgrades you can actually use in your mac pro, sadly. You can actually get a 12 core, I think, but I'd have to look at a video. Actually you can look at the video. lol.
Ended up getting the 10 core 2690. It was like $200 less than the 8 core, and seems to have a higher multicore geek bench score over the 12 core (interestingly). Kind of a sweet spot interms of cost to performance.
I'm glad I tried an ES, but honestly it was a buggy POS. I might have had a bad sample, but overall it just seems like gnats were rummaging in my silicone from time to time. Weird hiccups, especially with h.264 instructions. Virtual machines and development were rock solid, but anything involving paging or graphics heavy stuff sort of got tweaky. I sold the thing on ebay, for the next experiment.