Partly inspired by your thread, I ended up doing a project replacing most of the rear ball joints with the Lemforder 3067901 joints. I ended up a local group buy for these and the parts turned out really well.
Installed the rear M3 control arms and pressed in the Lemforder ball joints in them. The mounting face for the arms is tapered so I did a simple CAD design and we got the adapter washers CNC'ed out of 304 stainless. The ball joints pressed into the trailing arm bushing locations. The ball joints were pressed to depth taking into account the thickness of the custom washers
I already have the German Auto Solutions front monoball pressed into the M3 arms, and the rear end is now fully ball jointed apart from the inboard camber link. With this current setup I note that the rear end is a lot more stable and doesn't move around as much, less oversteer when I throw the car into a corner. Was suprised at the improvement even when not driving that hard.
Addtionally I note the rear end is more compliant/comfortable over slow/medium bumps and less comfortable on sharp bumps. The damping overall seems to be better without the rubber bushes twisting up and contributing to the overall spring rate.
