My boards give you 5V rated for 2A; the circuit should handle 3A output.
Other manufactures may be different, I have two different types of usb risers. One has a linear regulator and the other I cannot find a data sheet for, but I would guess it is a linear also. I also doubt the card pulls the full 10w at 3.3v. I will measure it when I get a minute.
You also can't assume that the riser board is designed to deliver all 10w.
I thought you "shouldnt mix" PSU´s when powering a GFX.
EX.:
PSU 1 - MB + 1 GFX PCI-e & Riser (by riser i mean the 4 pin molex)
PSU 2 - GFX PCI-e & Riser 2, 3, etc, etc.....
From what i read, it seams that mixing and matching PSU´s will cause undesired efects (Burn baby Burn)
Or am i just getting this wrong ??
I can confirm with 100% certainty you can use a server power supply to provide the power for the 6pin connections and the motherboard power supply to power the slots. I have built alot of machines this way. All of the power connections are isolated inside the card. You should never directly connect 2 power supplies together, unless they have a current share pin and it is connected correctly.
I used a 650 watt supply to run the motherboard and the risers for 5 cards. When I added one more it wouldn't run. I switched to an 850watt for 6 cards and works great. I use a 1570w dell supply to run the 6pin connectors