Srivatsa S. Bhat of IBM had presented at the Kernel Summit and then today shared his power experiments from his machine. First of all, his machine isn't what most people will ever experience: an IBM Power 7 system with four CPUs making up 32 cores and there's 128GB of RAM. With this system, testing out kernel power memory management had dropped his system's power consumption by up to 2.6%.
While this hardware is a bit extreme with 32 cores and 128GB of RAM, it isn't even an ideal system as there's only four memory regions for handling power savings. Srivatsa said there's prototype IBM hardware that can handle more memory regions and lower base power consumption where he believes the system power savings could be up to 5% with this feature.
For those not familiar with the active work on memory power savings by the Linux kernel, see Linux Kernel Power Management Targeting Memory. Srivatsa's findings can be found on the kernel mailing list with more details.