I have a Wiechers one on both the 156 and the GT.
The 156 has Eibach -30mm springs & shocks and the Eibach ARB. The suspension is wonderful, but initial drives were a bit fierce - lots of dash shake and rattles. Putting the strut brace in silenced all of the rattles. I don't actually know if it handles better, because it has been ****ing with rain ever since I fitted it, and I have gone nowhere near the limits of the car.
On the GT, it has also reduced the rattling, and cornering feels a bit more precise. However the GT is suffering from tired wishbones, so it is hard to be specific about improvements.