I don't know if there's a limitation to 75W per PCI-e cable. In my case there would be ~120W per wiring. Do you have a source for the 75W limitation? I'll look into the PSU reference guide.
I have been searching on Google and the general consensus is that a 6pin PCI can easily handle 150W. The difference between the 75W 6 pin and 150W 8 pin is just 2 ground wires. Some people even put the 6pin cable into the 8 pin connector of a GPU without problems.