You are correct in your reasoning, and I am with you in wishing that we could drop in any processor that "should" work. However, the HP workstation's BIOS includes a "microcode" listing of what processors have been certified for that box, and it is a general rule that if a processor is not in the microcode listing then it will not fire up.
There are some rare exceptions, but back when I was working a lot with the xw6400/xw6600/xw8400/xw8600 workstations I tried some of the xw6600's HP-listed 5400 series processors in the xw6400. No go.... not even a squeak from the boot process. They use the same sockets so I figured I'd try, and I did not damage the processors or the motherboards.
I did not try all the 5400 series processors, but I've never heard of an unlisted processor working in one of these two generations of workstations. Forum posters have found a very few exceptions to this general rule, with the Z400/Z600/Z800 series. For example, I'm running a X5690 in a Z400, and that was never listed in any of the QuickSpec documents.
HP generally has released a new microcode list rolled in with a new BIOS release (but not with each BIOS release), and I do know of one case where they released a new microcode list by itself (for the xw4600). I have never discovered a method to read that microcode list directly. HP does not talk about the microcode list concept hardly at all.
This is all related to the fact that these workstations need to run very reliably for enterprises, and thus HP needs to limit the processors to ones that they have tested extensively for compatibility. They can't have some IT guy throwing in faster processors that "should" work and have the enterprise blaming HP down the road for unexpected glitches that might show up.