I thought suspension springs were tempered? (Removal of residual stress fatigue once under load)
They are and the stiffness of the material is altered by repeated heating and quenching (sometimes in oil to reduce the cooling effect). If the metal is annealed too much it loses it elasticity and becomes too malleable (think of something like coat hanger wire).
I am not going to start to get into materials and structure and metallurgy (something I used to lecture on at university), it is a massive subject. You also have to consider that alloys and even things such as shot blasting the springs to add a thin layer that is work hardened and corrosion resistive.
At the end of the day, a spring for a car is designed to support the weight of the car and to allow a certain suspension travel. Some springs (like x-powers and the later red avos) are progressively wound so that as they compress they progressively get stiffer. Cutting a spring means there is less material to support the same weight, so the spring becomes overstressed and so can fail or fracture without warning.
The same thing happens as well with heating a spring. It will change the material properties and affect the capabilities, again allow stress fracture, stress corrosion, reduced (or just as bad increased) stiffness and early catastrophic failure.
My advice is to leave well alone and fit the right springs for the job rather than bodge a set.
