Agree.... that card should work in a Z600, but will it work to the USB 3.1 10Gbps speed standard? No.
The HP Z600 workstation has no PCIe generation 3 slots. It has 3 PCIe Gen2 slots, 1 PCIe Gen1 slot, and 2 PCI slots. Below is a link to the HP page describing the Z600's slots:
http://h20564.www2.hp.com/hpsc/doc/public/display?docId=emr_na-c01719695
Here is a note from StarTech regarding their card's speeds, emphasis added: "The maximum throughput of this card is limited by the bus interface. PCI Express Gen 1.0 max throughput is 2.5Gbps. PCI Express Gen 2.0 max throughput is 5Gbps. PCI Express Gen 3.0 max throughput is 10Gbps. The available bandwidth (10Gbps) is shared equally between the two ports."
These USB 3.0/USB 3.1 PCIe cards generally need power supplementation.... the +5VDC they need does not come up from the slot, but it is easy to find a 4-wire "molex" peripheral-to-SATA power adapter which can provide you the higher wattage 5VDC they need to meet potential power draws (versus what USB 2.0 needs). All this info came from research figuring out how to add the Texas Instruments based HP USB 3.0 card to our xw6600 workstations, which have 2 PCIe Gen2 slots (the rest are Gen1). Z620 gives you some PCIe Gen3 slots.... slots 2,4 and 5.
Here's the Z600 slots diagram:
