Fitted a set of B,4s on the back of my 3.2 Saloon couple of weeks ago.
The existing fitting kit that comes with the B4,s is not as substantial as the original GM kit,but I am not convinced it would give any real issues if fitted correctly.
Fortunatley I was able ,with the help of Nitro,to assemble enough bits to fit mine using genuine GM parts which when compared to the Bilstein stuff is quite evident why you would try and get Gm if possible.
The bottom rubber mount and the metal sleeve is required from an Estate GM kit but the top mount is the same on either.GM washers are also dished in shape meaning when assembled the top thread is long enough to get the nut and rubber mount correctly fitted in the boot.
My only observation after now driving some 1000 miles on the B4,s is they are noticeable harder in ride than GM shocks and as I tend to carry a boot full of stuff around with me it does have a tendancy to be abit noisier with more rattles.With 4 people in the car the ride is pretty good and the harshness seems to disappear.