ATT-TEC, a German company located in the city of Rastatt in Baden decided to optimize the convertible version BMW M3 through a tuning package that costs 26,573 Euros.
So here is what you get for that price: a G-Power compressor kit which alone costs 11,430 Euros and requires an additional 1,310 Euros to be installed, a sports exhaust system with 2 x 90 mm diameter stainless steel baffles (1,455 Euros plus 140 Euros to install), a set of 20 inch ADV.1 rims wrapped in Vredestein Ultrac Sessanta tires (7,150 Euros) and Inox Line III Variant lorry track coilovers that carry a 1,929 Euros price tag.
If you like what you hear then you should know that the ATT-TEC BMW M3 Convertible delivers a total of 520 horsepower and it also comes with with a special filming that includes carbon wrapping foil that costs 2,600 Euros.  

Source: ATT-TEC