I've driven great deal on tracks in various cars, including Sevens with fully adjustable suspenson setups, and Beedub, I agree with your sentiments regarding just sticking springs on a car and expecting radical improvements. Furthermore, as others have stated, BMW will have made far greater research investments into handling and suspension than Eibach or the like can hope to echo.
I changed to Eibachs because one of my rear springs failed, and I thought it pointless to replace it with OEM when they have a proven poor track record. I didn't want to lower the car or alter the handling characteristics, but I found it impossible to obtain non-OEM springs that did not lower the car to some extent.
However, having had the opportunity to drive my ///MC on the track with OEM wheels and stock OEM suspension (pre-spring failure) and then on the same track with the same wheels and tyres, but with Eibach springs, I am confident that the turn in is sharper, body roll whilst cornering is reduced, and power can be put down at an earlier point with the Eibachs. I've not driven on the track with my CSLs, but my impression from driving on smooth roads is that these would improve track performance further.
Also found the Eibachs to improve handling markedly - eliminates the "bouncy" first stage of the progressive OEM coils. Eibachs are also progressive but the transfer between stages is much less obvious and this makes handling more predictable. Stiffer bushes etc will improve handling even further but it gets costly and is a bit over the top for every day use imho. 
