Well as far as I understand suspensions, by preloading the spring you just make the ride rougher, or it may have no effect at all.

Unless you have linear race springs, most springs are progressive for comfort at light compression and gets much stiffer in a nonlinear progression the more it is compressed (beyond the linear increase). Your pre-compression bypasses the lighter comfort range. Since the weight of your car and cornering forces are the same for your car no matter what precompression you use, you aren't going to change the performance of the springs.

Alternatively, it's highly probably that you can't precompress the spring on the coilover farther than the static weight of the car compresses the spring... which means precompression has no effect other than making it harder for you to assemble the suspension for installation.