I think there is more to that, John. For a given ride frequency, the force exerted by the damper is a function of both the spring stiffness and mass of the body.
If memory serves me well the GS450h is about 200kg heavier than the GS300 (with most of this added weight probably biased to the rear). Installing the shocks from a GS300 may cause the car to be underdamped, as the shocks don't exert adequate force to dampen the movement (and thus dissipate the energy) of the stiffer spring and heavier body. May not be an issue if the car is normally driven solo, but when loaded with 4 passengers + a full boot, I'm not so sure...
Speaking from personal experience, a good chunk of a vehicle's development time is spent tuning the springs and shocks as a matched pair.