Things changed with the introduction of the Z600 version 2. There are hard-to-find HP DDR3 (and later DDR3 and DDR4) memory QuickSpecs, versions 1-6. Those show the ECC buffered sticks and ECC unbuffered sticks to be HP approved for the Z600 v2. For the Z400 v2 it stayed at ECC unbuffered. I'll add in some screen shots of the HP documents later.
1333 MHz is the fastest the processors for the Z400/Z600 can run at, but we know about the down-regulation of faster memory to slower MHz as set by the processor(s) present. Hence I experimented with 1866 ECC buffered server memory used in HP/Dell/IBM servers that is ECC buffered that works fine in that older v2 workstation. Not bad, used, at a bit over 11.00 USD/8GB stick. That was only after pro-level eBay searching, and it also works great in my Z620 v2 builds using v2 processor(s) running at 1866 MHz.