In multiple games i have seen walls that (can be added in map editor and) hides what is behind the wall, so if i stand in "front" of the wall, the stuff behind it is hidden, if i stand "behind" the wall, the stuff in front of it is hidden.
In other words, whats behind this wall is not rendered, untill the player gets to the other side of the wall. It would really increase performance if BI could implement this but im not seeing to this that much in arma 2, since it would be a huge patch changing the map and coding.
And example of this that i can give is FAR CRY 2, where the walls are.
Those walls could be put into hills and bigger buildings, so the stuff behind them wont get rendered. The only stuff that would be rendered is what you CAN see and HAVE to see.
No problems with Choppers/Airplanes because on a high altitude you can see OVER those hills (which also represent the walls) and view wouldnt be empty