i would strictly aggree with nismoboy on that one.Firstly it all depends what make springs you gunna have if you have a cheap spring say like pi or avo than the ride aint that good on the standard shocks and you will need lowered dampers but if you have h&r or eibach or even hks then you get a much better ride as they are better quailty.But one think i would say though is it is always best to put in a lowered damper in aswell as the the lowered springs as the standard dampers can start to leak as they are put under constant stress.
1) Lowering the car without installing shortened dampers reduces suspension travel. Springs like Eibach Prokits will bottom out pretty regularly. Forget Sportlines. They will bottom out a lot. High rate race springs with coilover sleeves, such as from Ground Control can be had with a high enough rate that if you have some disipline and don't drop it too much, you will be OK, but only with aftermarket, high rate dampers (shocks).
2) If you increase the spring rate you really should increase the damping of the shocks. Otherwise, the stock dampers cannot adequately control the oscillations of the spring - even an Eibach Prokit.
The bottom line is if you want decent handling and ride, at minimum you should consider KYB AGX with Ground Control coilovers.
Where can i find the Groundcontrol coil overs? Im intrested in lowering my P10, id like to still maintain a decent ride, and hopefully improve handling while having a lower ride height. A set of springs is relatively cheap, but I realize im better off saving some extra bucks and doing it "right" the first time. Damn I hate being the newbie!