Molex makes a crapload of connectors, many of which can handle current up to these levels, but the real question is: are they keyed to PCI-E?  And in the event they're keyed to PCI-E are the PCI-E plugs on the PSU rated for those power levels?  It's unlikely.  

The bottom line is that it adds negligible BOM and assembly costs to double up the PCI-E connectors on the boards.  Hell, even the original AM cubes had 2 PCI-E plugs and it only consumed ~320W OC'd.  Cutting costs and/or poor choices in something as simple as this makes me wonder what other kind of issues are going to crop up with the design.  People can sit here and argue until they're blue in the face as to whether it can or can't theoretically handle the power, but cheaping out and assuming that kind of liability is absolutely stupid.  If someone's house burns down, the sign-off Engineer can be personally liable for the bad design.  It's that simple.  If you want to take the risk of having FIVE of these potential fire hazards running in a closet in your house unmonitored 24/7 then be my guest, but I strongly recommend against it.