Would love to hear suggestions for cpu+mobo combo that is ultra low in power consumption.
You won't find another x86 CPU with as low a power consumption as an Atom and better performance, but there are plenty which can give better performance per watt with higher total power consumption. Tom's has had a number of 'Atom vs X' articles for various CPUs comparing performance per watt.
You probably want to look for the lowest idle power and then consider underclocking until the maximum load power is less than your limit. Even the i5s may be good enough if you underclock them, their idle power looks pretty good.