Hello, I have a 2014 A5 Cabriolet Quattro original wheels with
ALY58912 Audi A5, S5 Wheel Silver Painted #8T0601025CA8Z8 size: 18"x8.5 et29 with 245/40/18.
I replaced with (TSW Bathurst silver #2010BAT255112S72 size: 20"x10" et25) square - all around with 295/30R20. Fitment and stance perfect no spacer/ no suspension mods. Tires & wheels approximately flush with finders, about 8mm under from flush on rear and perfect even on front. You could still add 8mm spacers on rear to be flush on rear. Definitely would not go any wider than 20x10 et 25 with 295/30R20 if you are going square all around and without suspension mods and because you have a Quattro transmission requirements and need to be square diameters on wheel/tire around vehicle. Hope my experience with fitting my A5 helps some folks out there!