Factory springs shouldn't be cut yes but progressive springs which I have already can be cut, lot of people have done it even on coilovers.
I am not talking about suspension travel, the damper travel itself. Lets say that the valving in the chamber is sitting in the middle at normal height. Suppose you cut the rod 1 inch shorter then the valve will sit 1 inch higher at same height. Therefore if I cut the springs and rod 1 inch the valving will be at "middle"