Now can you please provide data for the maximum 24/7 safe current per 18AWG wire.
My own research led me to believe it is much lower than 9A.
Useful table here
http://www.powerstream.com/Wire_Size.htmusing "The Maximum Amps for Chassis Wiring is also a conservative rating, but is meant for wiring in air, and not in a bundle."
AWG 18 is okay up to 16A (~<200W @ 12V)
I highly doubt that you can run 550W through the PCI-E cable without any consequences.
One of my PSUs (Cooler Master V850) is powering an October Jupiter and each module is being fed by a different PCI-E cable and still they get noticeably hot (around 44 degrees max).
Or try to connect 2 modules per PCI-E cable and it gets even hotter, yet the current per strand is only around 8-9A.