The test is sadly but definitely true. If I might share my own numbers on a Sapphire HD 3870 (passive cooling, 512M).
The overall system usage at idle is 5W lower with fglrx on recent kernel (no pcie_aspm=force yet) than Catalyst on WinXP, and this is still about 5W lower than xf86-video-ati/Kernelside driver part. which makes it a 10W difference, idle. CPU etc. should be about the same so I think these 10W are purely GPU side.
(For absolute numbers I had to plug the power meter inbetween. Didn't notice that much difference (as expected) with just the onboard HD 3300 (64M sideport).)
So even on the somewhat older RV670 there is room for improvement. Still it is much better than my test some months before where I had the GPU running at full power/speed all the time with the free drivers and that was like 30W idle vs. 70W idle.
Stop TCPA, stupid software patents and corrupt politicians!