I have researched this. There was a member on this forum who tried it on a Single Processor Mac Pro and it did not work. I have googled it and can find no instance of the X5687 working on any Mac Pro. Further, I have researched the results ay Geekbench and can find no results of any Mac Pro using the X5687.
I did this, of course, because I in the market for the fastest 4 core CPUs that I could buy for my Dual Processor 5,1 and, I believe, the X5677 is it.