That hood scoop is a vent 90% of the time, and at WOT, the opening provides extra air volume for the engine intake.
At anything less than WOT, it keeps cool air coming in from the kidney grills and the lower bumper opening. Warm air is allowed to blowing out the vent under low to moderate speeds. (to prevent heat soak)
To the OP: If you actually close off this hood air intake/vent opening, I'm fairly certain that you find that it's a detriment to your performance. The intake system will retain heat (especially in the hotter summer months) that would normally vent off to atmosphere.
The hood intake/vent becomes a negative pressure (vacuum) suction source at WOT, and this additional air intake source supplement the air coming in from the lower bumper area, and the kidney grills. The factory air box/intake system is a very elaborate design, and it's very efficient at gathering a copious amount of fresh air to feed our 4.0 liter V8 engine.
This is why the OEM design is so hard to beat.
All you really NEED, is a high performance drop-in cotton gauze type air filter element to maximize the performance of the factory airbox the BMW engineers have already built for us.
Just my .02
