RD600 is certainly more power-efficient than 680i, but then so is a 3kW electric fire.

Okay, I'm exagerating: the RD600 chipset is quite power-efficient, but the gap between it and 975X is nowhere near as wide as the gap between it and 680i.
Of more significance, I think, is the fact that performance on RD600 is several percent slower than on other chipsets (including 975X) if both are run at the same clock speed. Overclocking on RD600 therefore is not something you do to
gain performance, so much as it is something you do to bring performance in line with what other chipsets already offer
without overclocking. And, as has been alluded to above, overclocking RD600 is
really complicated compared with most other boards.
And some 975X boards do overclock quite well: the Intel BadAxe2, for example. (Which also has a reputation for being absolutely rock-solid stable, and was specifically designed for quad-core CPus).
The DFI board that uses RD600 is a great
enthusiast product, but it doesn't strike me as something a "normal" user would want to mess with.