At the end of the straight at the Shanghai circuit where the car is travelling at close to 330kph with DRS open, the air pressure hitting the air filter will be close to 1,070 millibar – normal ambient air pressure is around 1,020 millibar. This means the engine produces around 5% more power – equivalent to 40 bhp – than at ‘walking speed’.
Interesting article here
http://www.racecar-engineering.com/articles/f1/technology-explained-f1-airboxes-filters/
I used to be very involved in a class of drag racing where modifications were limited and only normally aspirated engines permitted, in this world every single hp you make is important and air boxes were complicated things that required significant thought and experimentation