According to an official statement lowering an Alfa with the VDC system more then 2.0 cm is not allowed because the VDC can be a dangerous feature in stead of a safety feature.
This is why a lot of Alfa dealers give a negative advice about lowering. In case an accident occurs because of malfunctioning of the VDC, a claim will probably not be succesful (because lowering will probably void the warranty).
On the other hand. A lot of owners have lowered their Bella's more then 2.0 cm and haven't had any problems so far (apart from the negative camber, which is nice for cornering but bad for tire wear)