Got the same size on mine, 15mm spacers on the back for over three years now and nothing's fallen off. Yet. Traction control/DSC seems very sensitive on the Z3, when I had new tyres fitted on the rear the warning light kept flashing on almost every corner, soon as the new fronts went on it calmed down again. Bear in mind that the tyres are only nominally the same size, they can vary between manufacturers, not by much but enough to affect things like DSC, so if your 16s aren't all the same that might explain the light flashing.