Hi Digo , The most important is the balanbce of the car , what do you want , you feel a lot of understeer (your car don´t entry or go out of the curve (front axle) or your car have a oversteer (the rear axle slip a lot) , If you think that your car or for your type of driven the serial now is Ok , then you can change the springs with the same rate N/mm but lower then you will decrease the center of gravity if you reduce the length of the spring you will have less stroke of bump in the damper then put a shock absorvers with more bump to don´t make final of stroke, In my opinion I think that with a kit of Bilstein or Koni will work well.
For my experience is much better don´t increase the rigidity of the springs because then you will lose traction. If you give me more info I will try to give a good set up.