Here is my real world experience:
There is no doubt that the version 2 (with the later boot block date in BIOS) Z400 can run the X5690. I know this because I upgrade all our version 2 Z400 workstations to a X5690 and those have always worked just fine. I've also had access to multiple version 2 Z600 workstations. Any I've tried a X5690 in never booted. I have seen no trustworthy report of the X5690 working in the Z600 version 2 for years now. I know the Z800 version 2 can run the X5690 because it is approved in the QuickSpecs for that workstation. The guy you reference was untruthful.
The X5690 has never been officially approved for the Z400 version 2 but it works... for years now. I don't have access into the "microcode" section of HP BIOS to see the approved processors white list, but believe the X5690 is absent from the Z600 v2 and present in the Z400 v2's white list, but that for some reason HP never revealed that. The X5690 still works in a v2 Z400 with the latest BIOS.
You need to have a "Performance" heatsink/fan for the higher wattage processors to work in the Z400/Z600/Z800. They are cheap and easy to find for the Z400. They are rare and hard to find for the Z600/Z800. The expensive rare ones for the Z600/Z800 are identical. The one for the Z400 is different, taller. You can spoof a "Mainstream" heatsink/fan to look like a Performance one but this is unwise unless you just will be doing light duty work like email and surfing the web.
So, yes you can run the higher wattage processors in the Z600 but they need to be on the approved white list in the microcode, plus you need a heatsink/fan that at least has the same wiring as the Performance heatsink/fan.
The spoof: The CPU heatsink's fan has 4 wires coming from it whether it is Mainstream or Performance. The white fan plug end has 5 holes..... in Mainstream hole 5 is blank. In Performance it is occupied by a ground jumper from pin 1 to 5. Pin 1 is ground, pin 2 is 12VDC, pin 3 is RPM feedback from rotor to motherboard, pin 4 is PWM control from motherboard to rotor, pin 5 is blank or ground jumper. You can monitor your CPU temps in many ways, including watching for smoke.