Preload spacers or adjusting the preload threaded collars do nothing to stiffen the spring...even on progressive springs. If the shock is fully extended and you put spacers on to compress the spring you are not making it stiffer you are just increasing the abount of force necessary to get the shock moving. This is NEVER done on current cars.
On modern cars spring preload spacers/threaded collars are only used to set ride height.
What I run: Schumacher Mi5/Associated RC10R5.1/Associated RC12R5.2/Futaba/HobbyWing/Team EA Motorsports/BSR Racing
Where I run: Florida Indoor R/C Complex/Thunder Racing/Florida On Road State Series