I understand, that collision detection puts more load on the system and the grass is expensive even as it is now. But maybe there is a way to bypass this issue? Arma 2 laser rangefinder seemed to react to grass.
One idea is to run detection only, where a character is present. If a soldier is lying in the grass (that is if there are more than 4 grass bitmaps within 1m from him), then he is flagged as hidden from AI (under normal coditions, like not shooting at the AI). If a laser is used by a character lying in the grass, the beam is automatically cut at 1m length.