Happy to help. No, that was not brief!
The reason for the recommendation to use the Intel 320 series SSD is that it is a SATA II rather than SATA III drive. The Intel 535 is a SATA III drive, and HP has put out specific communication documents warning against using SATA III drives in its SATA II workstations due to potential for significant intermittent issues. Those topics have been posted about in detail in this forum. The point was that doing that was not a guarantee of problems to come, but that there was a significant risk.
The exception HP provided then was to use SATA generation III drives that are HP branded. It appears that HP engineers figured out the solution to the issues, and apply a HP firmware update to those HP gen III drives so they can work without concern in the older SATA Gen II workstations such as yours. In my work with the xw and early Z workstations I have followed this advice and never have seen an issue. So, if you want to use an Intel 535 I'd suggest making sure it is from HP. I've done exactly that, and it worked fine.
Finally, a SATA Gen III drive used in a SATA Gen II workstation will not go any faster than SATA Gen II speeds.