There is a practical limit to how far you can stretch the 150 Watts available on Mac logic board.
A guy posting at Netkas crossed the line.
He ran an MSI GTX580 that required Dual 8 pins on the Dual 6 pins. And it worked !!!
So naturally he booted into Windows and fired up an overclock utility and tried running those clocks up. As soon as he ran into trouble, he did the sensible thing and started using the software to increase voltages to the GPU core.
And now his Mac won't boot via his RAID card or on the GTX580 unless it sits for 10 hours.
So, I think we found what a bridge too far is. An 8 pin cable is designed for 150 watts, a 6 pin for 75 Watts. Use your own judgement.