Do you know what kind of power savings this kind of setup would have over your standard 5400RPM laptop hard drive?
That would all depend upon exactly which CF card were used. And not all makers give full specs on that.
But.. A typical modern mobile hard drive uses 1-2W when spinning, whereas a CF card might use perhaps 50mW (0.05W) max.
So, it will save 1-2W of power in continuous operation, less so if one normally allows the hard disk to spin down when idle.
This might add a few minutes, up to say 10-20 total, per battery charge on a notebook.
Cheers