MTM did the car and on their website it says that the car was striped of its paint and then polished and clearcoated. The plastic bumpers were removed and replaced with metal ones to complete the look. It is polished aluminium.
Looks fantastic though and with MTM's supercharger for the A8 could have 600+ bhp!!
There is a way to use silver-based paint on cars, which happens to be a very complex and expensive job!
Mercedes had a calender series with silver-coloured vehicles, unlike the above mentioned procedure they used very thin aluminum sheets to cover the parts - just for the photoshoot!
This car is also for sale for anyone interested. I was in Munich last week and the latest Auto Motor Sport had this vehicle in its classifieds at the back with the same picture posted, pic 2.