Back to the discussion on the hood vents.
Aero probably won't make a huge difference, but...
Having a vent there should dramatically increase the airflow through the radiator, which in M4 GTS has that, same with all the 911 GT3/2 cars. The engine bay airflow setup is pretty piss poor on our cars. All the high pressure air from the grilles through the radiator doesn't really have a place to go.
But I have to say it just won't look right on the smooth exterior non-M or M-sport e9x. You could have the bodyshop fill up the two vents on the side if you want to go with it though. Just FYI, most aftermarket hood has pretty awful fitment from all the pictures I have seen over the years.
You might want to message the OP on this thread, who went with a modded stock hood similar to what you have in mind:
http://www.e90post.com/forums/showthread.php?t=1172039
Btw, it would be interesting to see the data from the other side of the hood. Surely you can find a place to cruise at 80-90mph, no?
