It is not so much the difference in inertia as it is the reduced crank load due to the underdrive.
A graphic example would be, Stock pully is say the same diameter as all driven pullys (it isn't, this is an example). So, for every full turn of the crankshaft, every belt driven accessory also makes one full turn.
Now, the example becomes the underdrive crank pully and this time we'll visualize it as 1/2 the diameter of the original pully.
Now, for every full turn of the crankshaft, each belt driven accessory only turns 1/2.
So, you have doubled the mechanical advantage the crank has on the driven accessories, and it becomes that much easier for the engine to overcome the drag of the belt driven accessories.
Your small displacement, low torque engines in your past cars would show a rather dramatic response to this mod.
Low R.P.M. high torque v-8s don't show a lot of improvement as they have little trouble overcoming parasitic drag like this.
And since they are low operating R.P.M. engines, usually there are more downsides along the lines of poor a/c output, low amperage output, poor power steering response, etc. at low speeds.
And of course, if you're ever off road, you're not going 50 M.P.H. (I hope)
The Vibration Damper absorbs torsional vibration ringing through the crankshaft and prevents work hardening and ultimatly cracking of the casting/forging.
If there is a underdrive pully available, this would be incorporated into it.
Believe it or not, that was a short answer,