The announced visibility range matches the actual visibility range set in the scene.dat
Wether or not your crew actually sees something rendered in the theoretically visible space is a matter of the sensor settings, and you can easily make changes to that. For example at night it is very easy to overdo things and make your crew blind to ships that are in front of you.
The only reason that the detection matches the visibility is because modders have worked hard to ensure that, as it is highly annoying to be able to see yourself something that your crew (Of theoretically trained military lookouts keeping a watch) doesn't.
If what you want is to make aircraft less visible, play with the object size factor. Planes are smaller than ships, so you have a margin to lower the detection of them before the ship detection routine is screwed.
