AChambers378,
As you discovered, a Z600 with a boot block date of 01/30/09 supports Xeon 55xx CPUs only. A Z600 with a boot block date of 01/07/2010 supports both Xeon 55xx and 56xx CPUs.
An updated BIOS might have later microcode version, but the boot block determines what processors are supported.
Putting a 56xx CPU in a Z600 with the older boot block does strange things. The behavior can vary. Some systems will boot, others won't even show anything on a screen. Even if it does boot, it will be horribly unstable and unreliable, it can/will crash. I have never seen any stable combination of 56xx CPUs with an older boot block, and even it it booted I would not trust the system at all.