I just upgraded from dual e5520 processors to the x5677. There is a very noticeable difference in performance, even in just general usage it is far more snappy and responsive.
With the currently insane prices of the x5690 chips the x5677 are definitely an excellent choice assuming you don't need 12 cores for your workflows. 8 should be plenty for me for now.
These cost me about $70 each and then I used an excellent de-lidding service to get them ready for the 2009 Mac Pro that cost $130. So just $270 to double the speed of the CPUs, it has the same 3.46GHz core speed as the x5690 (which currently sell for about $350 each so $830 for the pair plus de-lidding service) so for me at least it will keep the Mac fast enough to last and if I ever need more multi-core grunt, hopefully in the future the x5690 prices will have dropped again so I can go that route or Apple gets their heads out of their asses and starts making some decent hardware again. I am mighty tempted by the new MSI vortex (they made a consumer Mac Pro, currently with 2 GTX 980's in SLI. If the hackintosh community gets this working with OSX....
I ran a 64bit geek bench test and the results are here
https://browser.primatelabs.com/geekbench3/5540209
Nothing shocking but good to see the numbers to quantify the speed.
Single-Core Score
2761
Multi-Core Score
20445
Now I did actually try to de-lid a CPU myself, I have done lots of computer mods in the past and watched a lot of videos of people doing it and no, it is not a good idea. I will write up a full blow by blow account in a separate post, but the tl,dr is I have a nice little expensive paperweight as well!
Ps, anyone want to buy a pair of e5520 CPUs?