2010S5CAB

New Audi convert with S5 Cab coming next week! I am converting the stock 19s to a snow set and picking up some 20x9 et40 Hartmann B7 HRS4 summer wheels. Now my understanding is that the stock wheels have an ET of 29, anyone know if this is correct? I'd like as aggressive a stance as possible without risk of rub so what should I use as spacers. I figure the new 20's with an et of 40 will actually pull the wheels in a touch. So how far can I go with spacers front and rear safely to get max stance? Not looking to lower car or anything like that. Thinking 10mm on front? no idea on rears. Will I run into any issues with spacers?

Thanks!