The "school of hard knocks" of lowering cars has taught me -
1. Variable rate springs are best IF you have enough suspension travel. Some linear rate springs act "nervous" when driving over minor (3/4") road imperfections. But so do wheels with not enough sidewall height. With enough travel...
Eibach makes so many springs, odds are they use existing units and are just experimenting with different spring rates. Rarely does a brand new spring need to be created.