In my rigs I use three unmodified risers and three where I have the +12V lines completely disconnected from the motherboard and running to a separate power supply.
The 3 powered risers are NOT connected to your PSU which is plugged into the motherboard???
Correct. That's why the lines to the motherboard are severed, unlike this riser which just adds a molex. With this one, you have two parallel paths from +12V to the PCIe connector on the graphics card, so it would have to be the same power supply...putting power supply rails in parallel with each other is...iffy.