I looked into this with my car as I find the rear end has a tendency to bounce on bumpy corners.
Both kevin Bird Birds and Chris Rositer AC Schnitzer guaranteed their work and would put car back to standard if you we're not happy.
Both where about the same price fitted. Both had test cars. I would pay them both a visit.
Looked at BMW performance a touch cheaper and the koni fsd kit cheaper again and oe replacement so would not affect insurance.
I would steer clear of anything that is adjustable. Theory is good, but the reality is you will have 1000's of combinations that won't be right and only one that works.
E90 330i SE Auto Graphite Grey Metalic