The lowering is quoted as 25mm to 30mm on their website. Anyway, I've ordered them now so let's hope all goes well.
Regarding changes to handling the website says that just changing the springs effects hangling in a negative way..
"Lowering the chassis by changing the springs
This is the cheapest way to lower your Alfa, you do this at the cost of top speed handling. Here we note the following; lower looks better but it is at the cost of handling, the lower you go the worse the car will handle, even if you use those "harder" springs, the geometry is just not made for this type of punishment, it will get its revenge on you sooner or later. Facit: This method is only for those who drive in the city or cruise at around 80 mph in the countryside."
I was told that the amount of body roll is reduced with this mod which surely is a sort of effect on handling? Then again, I was never any good at physics so what do I know..