Were you getting this range information from sensors or from your estimation, what was the aspect?
I don't remember seeing aircraft from 20 miles in LOMAC, but most of my dogfights start around 10 to 15 miles out. I do think it is easier to spot them in the game, when they are above the horizon, but not that much easier.
A lot of spotting an aircraft depends how you are looking at it. If your looking strait at the nose then you will not see it until it is closer, if your looking at the spine ( strait down from the top) then you will be able to see it from farther out.