they should
but if low power is the goal then installing just 1 of these http://www.cpu-world.com/sspec/SL/SLANM.html will get you down to a 35W TDP at a great performance penalty of course (there is the L5215 as well with a 20W TDP however this CPU does not have a C0 counter part sadly)
however its worth noting the MP3,1 as a platform ignoring CPUs draws quite a lot of power
you may be better off with a low power Mac Mini or if you need a Mac Pro find a single CPU MP4,1/5,1 and kit it out with DDR3L and a low power CPU
Thank you for your informative reply. Do you mean using the Mac Pro with one cpu only and leave the other cpu socket unpopulated?
Yes I know it is a power hog anyway this is why I've removed the dual DVD recorder box and swapped the original 8800GT for a 19W fan-less efi-less nVidia GT710 Kepler card but I still get 150 Watts at idle.
Could you perhaps suggest a 4,1 or 5,1 configuration in detail and perhaps a source? Yes it has to be a Mac Pro so I can use audio / music software with at least one PCIe audio card.