Just to be clear, not talking about which GPU is the most economical watt/hash-speed
As far as the ability to tune down unnecessary pieces of the computer, as well as undervolting your cards successfully.
I know using windows 7 and MSIAfterburner I have been able to reduce power consumption by about 10% TDP from the wall.
Although I am experienced with using linux (debian flavored) distributions for my day job, I haven't gotten into GPU Mining with linux due to the problems with AMD drivers and the different ubuntu versions. (Ex: Ubuntu 14 LTS vs Ubuntu 16 LTS, some of the 'older' AMD cards aren't supported in the Ubuntu 16 version)
Leading to my question: What operating system and tool would allow me the most control over EVERYTHING that consumes power for the whole machine, and be able to shutdown unnecessary processes while still maintaining a stable machine?
Soft question: Is the difference in lost hashing speed with a linux distro (over Windows installations) able to be made up by having a super low power draw?