If you want that ghetto look of down 60mm+ you might have issues.. Lowered springs often have a soft ride.. Soft ride is unstable and provides too much movement in the damping effect of the suspension.. Therefore the handling goes out the window and the car will roll around corners as well as struggling over bumps.. Also it alters the angle at which the driveshafts sit at, and can cause them to give up very quickly!
When lowering say 30mm, often the springs are more sports orientated, with harder springs which compress less.. These improve the handling around the bends, but will leave a harsh ride which will have you seeking new denchers every so often
For performance, ideally you need a balance between low centre of gravity, and stiffness of springs (the compression rates of springs are measured in lbs - ie. 180lbs).
I hope this helps as a general guide, but I'm only going on my experience with Saxo's.