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What are the additional 2 wires on the 8 pin then?
They connect directly to the ground. It allows a PCIe compliant device to perform a ground sense to ensure the proper connector has been attached. All the power on both a 6 pin and 8 pin connector come from the exact same 3 12V pins.
To be accurate the power comes from the difference of potential between the 12V pins and the ground. If you consider what is transmitting power (electrons), they are in fact coming from the ground pins into the device and going out of the device in the 12V pins (electrons are attracted by higher potentials).
What defines the maximum power is the total current supported by the 12V pins and the total current supported by the ground pins. If the minimum of these two is below what you need to power your device, smoke will happen.
As the 2 additional ground pins on a 8-pin PCIe connector are connected to 2 of the 3 other ground pins they don't rise the power available. The 3 existing ground pins and the 3 12V pins having the same properties DeathAndTaxes is right in practice : what matters are what the 3 12V pins can handle.