x1 is fine for mining. PCI-extenders just use a ribbon cable to allow you to raise your card from the board.

Application-specific integrated circuits, or ASICS, would be much more energy efficient for mining. As far as I know no one has done this yet. Field programmable gated arrays, or FPGAs, have been used. These also are highly efficient but can be prohibitively expensive... though cheaper than an ASIC.