I would suggest looking at a mild upgrade like a B12 kit from Bilstein (Eibach Pro-Kits with B8 sport dampers), about 10-15mm lower than M-sport. I have this kit and added dinan camber plates at the front which lift things up by 5mm, so it's not an agressive drop.
Similarly you could look at grabbing BMW performance springs, they are at most 10mm lower than stock, and pairing with B8s or Konis.
If you really don't want to drop the car at all then either Koni yellows or Bilstein B8s (or even B6s) could work.
While in there remember to replace all the small rubber bits/spring pads, shock mounts/strut mounts etc. And then you might as well do the rear subframe bushings....oh and brake lines since you have to drop the subframe...
Sheesh...once it begins....
