I had the 'crashy front' and still do even with the new Koni FSD and Eibach spring combination. All control arms, bushes, and droplinks are now replaced. It all made little difference apart to get rid of a few knocks. The problem is a shuddering feeling in the front of the car, a bump sets off a shaking that oscillates a couple of times. Body control is fine, corners really well, feels balanced... but just can't go over a bump in the road without the shaking and the body flex (steering wheel vs. seat vs. floor). Maybe my car is missing a load of welds or something...
The root cause - I suspect the unsprung mass is really high on the 156 - large discs and calipers, a cast-iron upright and lower control arm, thick driveshafts and big CV joints, the upper arm, AND the weight of the strut, and that's before you bolt on huge wheels and tyres
The upper arm and strut are probably inconsequential - their weight seems low relative to the other parts.
I'm using GT 17" wheels/tyres which I think look lovely, but they are so heavy at 20kg each. If only there was a way to lighten them and reduce the unsprung mass that wobbles around every time the 156 hits a bump. I'm pretty certain that the only way to reduce the 'crashy front' will be to get the unsprung mass back down to 80's car levels (you know, when cars had tiny brakes, pressed-steel McPherson struts (rather than cast iron), and 14" steel wheels weighing 10kg).
Carbon-ceramic brake discs would be a good start (if only they were available), alloy brake calipers like the old days would be good... and is it possible to reduce the weight of that upright (spindle) in a safe manner, by grinding material off it, for instance?
The ratio of sprung to unsprung mass is what determines the ride quality, but I don't think we want to increase the sprung mass that Alfa Romeo tried so hard to reduce with plastic engine parts, magnesium seat frames in the early 156s, etc. Having said that, you can see why the V6 versions always ride better than the Twinsparks - the unsprung mass is the same, but there is more sprung mass to maintain an even keel as the unsprung mass moves around. Basically, every moving part in the suspension needs to be made lighter, but with the 156, it seems that Alfa Romeo made every moving part heavier - and then made the not-moving parts (such as the rear crossmember) lighter. I think this is why the ride quality is so much worse than the old 155 or even the 75.