The thickness of paint can vary between manufacturers and even individual vehicles, and although 70 microns is considerably lower than the rest of the vehicle it is within a normal range. There is no recognised minimum standard for paint thickness, but it would be interesting to know if other examples of the A5 Sportback also have paint of this thickness around the bonnet area. It is possible that the thinness of the paint has contributed to the stone chipping, but unless you can evidence that this is exceptional on your vehicle it is unlikely that the dealer would be willing to repaint it free of charge.