I have of course read the xw8400 Quickspecs but that is specifications of what you could buy a xw8400 preinstalled with, not a list of supported CPU. And the Quickspec was released in feb 2008 and the first Xenon 8400 was released after that it could not be in the list of CPU you could order at that time from HP.
And since the Intel® 5000X can be used with the Xenon 5400 series I thought it could be possible with a BIOS update to run that on the xw8400 motherboard. But a updated list of supported CPU is nowhere to be found.