I know; I only mentioned this difference bcause of your phrase:Damping valve can be easily removed.
I think that if you want to modify damping, this has to be done with the electric (electronically controlled) damping attenuators in the sphere-supports.... so you can play with pressure, maybe with damping too...
Why change the Toyota spheres if they also still are good? IMO it´s easier to fit a valve than adapting the thread.Because many of them can be used from Citroen C5. They are still good after all those years and can be bought cheap.
Actually, this is something I never thought about; the spring rate is ok for me, I don´t do heavy offroad; I like the car as comfortable as possible for on-road use and able to go further when the road ends.... it's the only effective way of changing the rate of the front suspension on AHC vehicles to anything between the factory specs and the much higher resulting rate of installing non-AHC torsion bars.