Replacing your factory springs with lower springs and keeping your factory dampers is a bad idea. With 55k on your car, your original dampers are already on their way out. Installing a lower spring will only accelerate the decline in ride quality. If you want a lower car with improved handling, do it the right way. Replace the springs/dampers all at once along with FCABs and RTABs and whatever else is worn out.
Past: 1999 Techno Violet Metallic M3
Present: 2012 135i DCT