Lowering springs seems to be a common mod and a big area of discussion amongst the E9X owners so just to help people out and summarise:
Eibach Springs (prokit) - progressive spring rate. Generally accepted to be slightly more comfortable than HR. People usually experience a drop of ~30mm.
Eibach prokit have a spring rate front/rear of 148 lb/in and 456 lb/in respectively.
Compared to stock E90 Sports rates front/rear of 145 lb/in and 460 lb/in.
So they are very similarly weighted however offer a drop of 10-15mm more than the Msport height.
HR Springs - linear spring rate. Generally accepted to handle slightly better than Eibach. People usually experience a drop of ~30-35mm.
I installed a set of Eibach springs on playwithonehand's car... the below pics show Eibach VS standard non Msport.