It reduces the upper area of the wing probably less than a percent, and even still the high pressure field of the wing is very tall, taller than 950mm, so the pressure acts on what little surface area there is above the duct as well.
The bulbous part adds little friction, in-fact less than we think becuase the bulbous part is shadowed by the air inlet, which has a larger frontal area. This is how well the Mclaren f duct is designed. Other teams have ram ducts sticking out in open air, which is a compromise.
About the adjustability, i think the team fabricates multiple end pieces, maybe a piece for every angle change they plan to use. That's a small price to pay.
The gain, as i mentioned, is like having 2 different cars on the track at once. The little milli newtons of drag, and grams of carbon, are an afterthought.
The beautiful thing as well is that no matter how other teams say they are building an f duct, it will never be controlled by the driver like in the Mclaren, and it will also never be as unobtrusive to the aerodynamics.
I feel amost sorry for the other teams when abu dahbi, Brazil and Monza come around. Mclaren dont have to compromise a setup for the interlagos back straight.
