No offence, but if you think the factory suspension you have currently with 18s are a little bit on the harsh side, then fitting adjustable coilovers will only make it worse. No ifs or buts about that.
Also if you are planning to go 19s or 20s then I can tell you now your ride is going to be worse that what it currently is as you will be running lower profile tires which means less tire to cushion your ride. Spring rates for adjustable coilovers are always going to be stiffer than factory ones and the lower you adjust them, the harsher they become as well. I am no suspension expert but that is just from my personal experience.