Skylarking and OP,
The Z620 power supply is easy to release and rotate out so you can see the amperage label on the top for the different cables.... those two auxiliary PCIe power cables the OP showed in his original picture are "G1" and "G2" both on the plug end and the power supply label. Each of G1 and G2 are rated on the power supply specs label at 18A 12V. That's a lot of watts worth of power available through each cable if you do the math. I'd never run them at that max, of course.
So, the capacity before you blow a fuse in the power supply is well over the ATX standard of 75 watts for each auxiliary PCIe video cable. In fact, there is a Z workstation instruction sheet for high power video cards that shows HP included an adapter with some cards that let you split one of their 6-pin auxiliary PCIe power cables (G1 or G2) into two 6-pin plugs for a card that needed to be fed power that way.
Yes, you are right that HP does not follow ATX standards..... these are not consumer grade workstations. They are engineered to higher levels.
I would not hesitate to power up that card with those two 6-pin plugs, and even would not hesitate to power it off one of those plugs if I had the HP 6-pin splitters noted above.
