I bought a quad core mac pro about a year ago, and have since started more virtual machines, and more video rendering. I think the machine could use some more cores. I think I am pretty comfortable swapping the cpu on the machine, as I've done mac mini repair as well, and system building.
I saw this, which is fairly cheap
How do you think it would compare to my (current cpu) quad core - 3.7 ghz Xeon e5.
I've bought 3 E5 2630L Xeon's in the last 6 months though only @ 1.8 GHz being ES CPUs. Good enough though for my uses which was enable X99 boards to function as they have multiple PCI-E slots and high reliability running 24/7.
The Intel pro-sumer multi-core processors would have been overkill and much more expensive. Though I've got 1 5820K for gaming of course!