Technically, the term "preload" only applies when the fork is topped out. At any other point, the spring force is balanced by the load (weight) on the spring. When you add a spacer to the top of the springs, it ALWAYS affects ride height by the amount of the length of the spacer. For a given load, the spring length will ALWAYS be the same.
For a more aggressive feel, go with straight rate springs. Progressive rate springs are an easy way for manufacturers to make a bike that will "adequately" handle a broad range of riders but they are not "optimal" for anybody.