Using less than 8 cores - does it save power?

Intel processors have had a feature called SpeedStep for quite some time. It dynamically down-clocks the CPU(s) during idle and lighter loads to conserve energy when maximum performance is not needed.

I wouldn't be too worried about the average power load of a Mac Pro. Unless you're doing something very processor intensive (like rendering video), you won't see anything close to its 1,000W maximum draw. During average use, it'll probably hover between 150-300W, depending on what video card you have, number of hard drives installed, etc. That's comparable to a medium-sized flat panel TV.

 

Unless you're using those cores all the time I doubt it will make a noticeable impact on power usage, processors don't draw excessive amounts of power when they're not in active use. Might actually be cheaper to keep them on if you do heavy tasks, that way you're running your whole machine for half the time vs running it twice as long with one processor short