If you want to get the most out of FSDs you probably want to stick with stock springs. Due to how they function you might not trip the "soft" valving unless there's enough bump travel. If you're not running bumpstops shortened by the amount the springs lower the car the valving may always use the firm valving. Then if you do run shorter stops they will have to be far more firm, so big bumps that use the soft valving will lead to a crashy, harsh ride because the stops ramp up the rate far more quickly than the stock stops.
If you want to go a bit lower and still want a smooth ride your best bet is probably a set of Neuspeed Bilstein Sports, and if that's still not soft enough you'll have to find a tuner that can revalve them to suit your needs.
Thankfully Bilsteins are more or less designed and supported with this kind of customization in mind.