When discussing inertia in a keyboard it is important to note that the inertia of the hammer transferred though the action and appearing at the front of the key is much larger than the inertia of the keystick itself, even with lead weights in it. So lightening the keystick is not going to make a big difference in total inertia. That said, Terry is right that a more massive keystick will take a little more finger force to get moving that a lighter keystick, regardless of how the key is balanced. The downweight is constant at all speeds. But the inertial force depends on how fast you try to make the key move. Playing very softly you might not notice a high-inertia key. But you will notice it when you play fortissimo.
As for dynamic range, I have heard that the high end of the power is limited by flexing in the key and the action. The front of the keystick hits the keybed while the hammer is just beginning to move. Once that power level has been reached, pressing the key even harder will not change the power in the hammer, because the only thing pushing the hammer is the unwinding of the flex in the system. Maybe Don Mannino can confirm this from his high-speed photographic studies?
Robert Scott
Ypsilanti, MI
This PTG archive page provided courtesy of Moy Piano Service, LLC