i asked the same question few time ago to Jhon Thorne from thorneymotorsport...
that was his answer:
I'm afraid that there are no really good lowering kits for the CSL, the Eibach kit actually raises it. We'd recommend a suspension kit like KW, its not a question of waiting until yours have worn out, the KW's really do improve the handling hugely.
i understood there was really not other solution to feet a good suspension kit, (CSL is not a car for second range upgrade), so i bought Bilstein PSS10, the last model of suspensions develloped on nurburgring in end of 2008 for M3 E46 and CSL.
i had just a word to say: FANTASTIC!
if suspension of CSL are quite good with double effect in sach dampers, and specific spring (i could compare with my ex SMGII M3 mounted with eibach specific spring with 20 mm lowering), the BILSTEIN parts bring your car in another dimension...the performance step is about the same between M3 standard and CSL, and between OEM suspension CSL and Bilstein. As we say in french: "un monde" (the world)
Now i can lowered my car as i want with something manufactured to work in those conditions. More, i can easily access and adjust the damping force with 10 level...
it's not to compare bilstein, or else with another mark, but just say that this investissement is problably the best ratio money/result to increase performance of your CSL. Do it if you can, you will not regret!