My son had this exact thing done on his M3, except the cutout is just before the mufflers. It cost him $500 total including the valves and switch, including labor to install and weld the valves in place. He did the wiring himself. Sound is awesome open. Better than my $7000 aftermarket system on my M3.
That being said, it does not create any horsepower like my system does. My full race exhaust system gives me about 35 HP. We dyno'd our cars with valves open and closed, and there was 2 HP difference on his car.
2015 i3, 2016 488 GTB
2014 911 Turbo S, 2014 X5 3.5