I figured it's the amount of energy needed to cock the spring that determines how much energy the hammer has. A lighter hammer will simply be accelerated to a higher velocity resulting in nearly the same energy.
I see a lot of Ruger Old Army cap and ball revolvers sporting lightened hammers in black powder target competition. Usually drilled full of holes like swiss cheese. I think bobbing the spur would be more effective since the mass of the spur is so far away from the pivot point, but then you'd have no good way to cock it.