When you mean upgrade... Lower? Same height but better?
The Eibach springs are meant to be a great upgrade but will lower the car by about 30mm, unless you have the factory lowered in which case 15mm.
My Z4 has the factory lowered suspension (15mm) and it scuffs on some of the local speed bumps. I also beached the car on a raised manhole cover at my local petrol station. Oh the sound... and the looks I got
Always found it handle really well, even with one of the rears snapped. Really didn't fancy losing another 15mm.
Just checked your car details on your profile and it says 2003 3.0i. That should be the 15mm factory lowered m-sport fitted as standard. IIRC it was fitted to all the 3.0i models at the start of production.
Sorry for the delay in responding. Yes they do remove the crashy ride, but only to a certain extent. Don't forget you are still riding round on 40 profile tyres (35 rear), but the ride is much improved. Not spoken to anyone yet who has changed that regretted doing so.
My other issue with run-flats is that I had a puncture on a rear, and the car drove pretty well. So much so I didn't notice it was punctured. Was only when I gave it the beans off a roundabout that the back end squirelled a bit and the traction control came on. Tyre looked ok from visual inspection, was only when I put a guage on it that I knew it was way down on pressure.
Result is that it destroyed the tyre side wall, and it possibly could have been repaired (yes I know they say you can't). This was when I started looking at the standard tyres.
Bought myself a BMW tyre inflation kit (pump & tyre goo) just in case the worst should happen!