The question how far can you see another aircraft is a difficult one to answer as it differs whether we speak of a pedestrian looking up in the sky a an airliner, of a pilot looking at a runway down below or of a pilot looking in the sky for another plane. This last case is the one of interest here. the information available on in this is quite interesting:
"Human beings and other species depend upon a number of sources of information, widely referred to as depth or distance cues, to judge the distances to objects. Some of these are physiological depth cues, like the convergence of the eyes as objects approach the observer, changes in the shape of the lens for objects at different distances (called accommodation) and stereoscopic depth perception. The range of distances for which accommodation and convergence are effective is quite limited, only a few meters (Hochberg, 1971). Stereopsis may function to a distance of 1,100 meters or so (Gogel & Mertz, 1989).
US Army research has shown that the average Visual identification range among trained personal can reach an average 1270 meters.
"An aircraft at a distance of 7 miles which appears in sharp focus with the foveal center of vision would have to be as close as 7/10 of a mile (= 1126meters) in order to be recognized if it were outside of foveal vision." Federal Aviation Administration Airman's Information Manual (AIM) Section: 8-1-6 Vision in Flight
It seems, indeed, that the logical conclusion is that, in Il2, we need to allow planes top be seen from a distance of 1200 meters. Whether this is currently best done by using a single LOD1200 instead of the present LOD500, or by adding a second LOD1200 to the present LOD500 must be answered by trial - the criteria being how this influences the fps and the esthetical perception.