I don't doubt Eibach etc can make good springs or dampers, but it's integrating them in the package that is pre-defined by BMW.
There are a large number of suspension elements that already exist in a defined state set by BMW, so just replacing one of them that isn't exactly the same as BMW's, introduces a change that impacts all the elements of the whole suspension on the entire car!
To suggest that any given suspension manufacturer considers the whole package when doing this is nice to believe, but it just simply won't be the case.
Anyone who owns a 3.0i or sport set up Z4 will already think it's boderline too low, so lowering more is just madness for road use.
Personally I'd go for quality fully adjustable aftermarket dampers, new original bushings all round, and keep the spring rates as standard (pretty good imo)
A spring is easy for BMW to get right, relatively speaking, a damper is often skimped on wrt cost, and is what can offer the best improvements to a cars handling for road work!
All different strokes for different folks, but suspension is a fine art, everything is a compromise. Nothing I've ever driven that was modified wrt suspension on the cheap drove half as nicely as good original kit!
Dave