Thanks for the links, some interesting reading.
Probably the B85 would be the safest pick as vitconik has one in his signature and getting 14W idle, so fits the range you are looking, not sure the other two, probably the Z87 will be less efficient with the VRMs oriented to overclocking.
Yeah, despite the shenanigans with the MicroServer it's still good to see what other people managed to get rather than taking a chance. All the review sites don't really test ultra low power operation properly.
Now one important thing to note is that you might be able to drop let says 15W for a headless unit or less (depending on the components), but the reality is the 3.5 hdds will increase the the idle by 4W each, unless you can sleep them (i never could under whs on my HBAs), so how important going for an extreme low power setup, will matter less once 3.5 mechanicals enter the equation.
I will report back how much difference they make. I didn't do anything special to get sleep mode working, I just enabled it in Windows 7. The drives are spun down about 90% of the time or more. When I browse the DLNA server from my TV there is sometimes a few seconds of delay while a drive spins up, but otherwise it works flawlessly. I have yet to find a mobo that doesn't support it, so perhaps it is a WHS issue.