I read the OP but not all subsequent responses so if my comments have already been mentioned or addressed, my apologies.
First, I appreciate the approach taken for realism. Obviously for those of us who have not flown actual combat missions what one sees and experiences can only be imagined.
Second, for me personally, using only the game's graphics, spotting is as follows. On the clear horizon I can see contacts at 9.5km if I know where to look, otherwise I can generally pick them out pretty well at 8.5km. Spotting planes directly below me over forest is much more difficult even if they are within 2km. It is what it is and I accept that.
So the spotting within the game I have no complaints with. Some may use third party software to enhance their viewing but they are still restricted by the 10km sighting range. There are however two points about plane visibility that I would like to make that I KNOW are not realistic and take away from the game's immersion.
First, invisible planes. They exist until they take some action when they become visible and it has been documented by myself and others as far back as 2015. Having an aircraft in clear weather 30 meters from you than you cannot see should not exist.
Second, and this is something fairly recent, is disappearing/reappearing planes. Do not confuse this with planes going in and out of the 10km sight range. I am talking about watching planes at 2 to 3km just vanishing and then seconds later reappearing. No real life condition explains such behavior.
Thank you again for all your research on spotting and trying to get IL-2 as close to reality as possible. I hope my two concerns can be addressed.