Generally agree with the advice to replace the dampers first then assess. Can't judge Bilstein vs. Koni but I will tell you - if you choose Konis - to buy the Yellows and avoid the FSDs. The FSDs are a comfort-oriented shock and are very soft in their initial travel. It's not a good match for our cars.
2009 128 Coupe / 6MT / Premium / Sport / Xenon / M3 Front Control Arms / M3 FSB / M3 Rear Subframe Bushings / Koni FSD / CDV Delete