Mac Pro CPU Upgrades
The Mac Mod/Upgrades page has Mac Pro CPU Upgrade reports and some for Xserves.
2013 Mac Pro FYI: The Late 2013 Mac Pro's CPU can be Upgraded - it's still socketed and as of early 2014, it's been confirmed the CPU can be upgraded. (Page updated with info on more DIY 2013 Mac Pro CPU upgrades, benchmarks, compatible Xeon CPUs and a Video with step-by-step guide on how to replace the CPU in a Late 2013 Mac Pro.)
2009 Mac Pro FYI: See Notes on 2009->2010 Mac Pro EFI (Bootrom) Updater (w/6-core CPU and 1333MHz RAM support) for notes/tips regarding the "unofficial" EFI/BootRom updater for 2009 Mac Pros to flash a 2010 Mac Pros Bootrom. Post also includes link to 2009 Mac Pro CPU Upgrade article.