KevinM These details are becoming more interesting.
First, we do need to consider static down weight separately from dynamic weight. Static down weight is minimum weight need to move the keys just a little bit. This should be around 50 grams. My MP11SE measured about 55g, whereas my FP-30X 70g. I'd say GF1 gets the static down weight about right. Vidal should have the same static down weight, otherwise it would feel off to players.
Static down weight is about balancing the seesaw, therefore it can be changed by adjusting the counterweight. If one end of the action is heavier, adding counterweight to the opposite end will help restore the same static down weight. However, increasing mass on both side of the seesaw increase the moment of inertia. Think a seesaw at balance but with super heavy weight on both sides. Because of the balance, it still only takes a tiny force to begin move off balance. But because of the super heavy weight, it will take a lot of force to move the seesaw fast.
If you look at GF1 but only the part between balance pin and front (player side) of the key stick, the shape and mass of this part resembles the acoustic action more closely than Vidal which has that extra weight plate. Because the action is balanced, the torque (center of mass x length of leverage) on the action side should be almost the same (with the static down weight offset) as the player side. If GF1 action's capstan is at the same distance as the capstan in acoustic action, then the mass of its simple hammer must weigh about the same as the acoustic's whippen & repetition & hammer assembly.
But then how could GF1's dynamic weight feel lighter than acoustic action as you described? I suspect it lies in fiction, or lack of it in GF1's simple hammer mechanism. There are a lot of hinges in the acoustic action that frictions could add up to resist moving the keys fast. This means dynamic weight is the combination of momentum of inertia and friction. Maybe Vidal's purpose of adding this large weight plates is to compensate for lack of friction in these simpler seesaw actions?
