Molex (company making the pci-e power connectors) dictates the max spec per pin to 8A according to an earlier posted PDF, so 400W is outside even the most optimistic estimates! Not saying it wont work, but why are they even gambling with this when it would cost them a few bucks tops to use 2 of them.
OK, so 6 pinsx8A=48AX12=576W per each PCIe cable max?
To me it sounds a bit too much, or maybe not each pin (out of 6) counts?
I am very far from being an electrical engineer, sorry.