[skidlids]

Preloading a progressive spring can make a difference as you should move the setting futher along the curve that relates to the spring rate.
But without changing springs you ideally need to increase the amount of travel available which is what preloading will do. putting it back up the curve leaving more to play with as the biggest factor is available travel

If you haven't got the correct springs for your weight and style of riding then forget about the ideal sag figure

By adding say 10mm of preload and therefore reducing sag you will have another 10mm of travel to help soak up the forces, if the average rate of a progressive spring is 0.75kg/mm that extra 10mm of preload will allow for 15kg of load to be applied before they bottom out.

If fork travel is 130mm and the ideal spring rate for you is 1kg/mm then with a static sag of 30mm it would take a load of 200kg to use up the remaining 100mm of travel.
If spring rate is 0.75kg/mm it only takes a load of 150kg to bottom out the forks, but if sag is only 20mm then it would take 165kg to use the 110mm trave and bottom the forks, not the ideal 200kg but closer to it