As I am an old guy who used to turn my own wrenches, but nowadays just don't have the time to do large projects, this post regards what I've had done to my car and how it now handles.
I bought my 2004 330 Ci ZHP convertible with 6MT transmission with 107K miles last February. Since then the tires have been changed from Yokohama 225s on all four corners to staggered Continentals (225 front, 255 back). It was after the tire change when the suspension issues become clearly felt. The car had a troubling porpoising in long, sweeping high speed turns as discovered one night on our local interstate merging lane (I-74 to I-73).
The suspension was completely re-bushed with OEM rubber bushings, the Koni psds were swapped out for Bilstein Sports, the front control arms were replaced and the rear trailing arms (the primary porpoising culprit) were also fitted with tab limiters in addition to getting new bushings.
The outcome is the car now has excellent handling qualities. The ride is firm but comfortable. This work did screw up the steering sensor. Alignment, which usually clears up steering sensor issues after suspension overhauls, did not work and a new sensor had to be installed. We are now good for many miles of happy motoring.