^^^^I can't really answer your question, but what I do know:
1. The X5687 is a fast (3.6GHz Quad Core chip of the same family as the E5620 (also a Quad Core Chip, but much slower). The E5620 was OEM in the 2010 base Dual Processor 5,1 Mac Pro.
2. I have googled the X5687 and the Mac Pro and can find no one who has used this chip in their Macs.
3. In reading the specs for the X5687 and the E5620, they are very similar (as I said, same family) and IMHO, it should work in a 5,1 Single or Dual Processor Mac Pro. But, as I also said it's never been done.
4. If I were you, and you want a fast 4 core chip for your machine, I would return the X5687 and buy either a W3580 or an W5590. Both are 4 core, 3.33 GHz CPUs and are known to run in a 4,1 or a 5,1 single core Mac Pro.
The Valley benchmark posted above by funwithdesign are for a single W5590. The CPU line indicates the W5590 chip and the last entry on that line, x8, indicates a single CPU. If it were dual W5590s, that indicator would read x16, as mine does.
Lou