I'm also concerned with backwards compatibility. I doubt a GPU like the 750 Ti for example would like a 300W slot.
Unless the PCIe slot pinout has changed, a 750Ti will be perfectly fine in a 300W slot - the card only draws what it needs. Where compatibility issues will arise is trying to put a 300W card in a 75W slot since the motherboard most likely won't be able to handle that.
If sending 300W to the PCIe slots require having a bunch of extra 12V and GND cables going to the motherboard, might as well stick to 12V connectors on the cards themselves.