We also use @alive's function to restrict the total number of AI physical spawns, regardless of how many are virtualised.
So it is
not necessarily true to say that if every area on the map was occupied by a player/UAV that all AI would be spawned and that the server performance would suffer. This is in reference to military and civilian objective module placed units; CQB units do not have a spawn limit, but as they are handled by the headless client, it's not such an issue.
In the example of players in multiple heavy AO's, what does happen which can be a problem, is that as AI die they are replaced, up to the spawn limit, by virtualised units which suddenly pop into view, possibly at close range to players. However, for BEST server performance it is wise to stick with one AO.
Pilots need to be aware that on some missions there are also non-Alive controlled permanent anti-air units placed on the map in strategic locations to make their life more 'interesting'. These don't just pop up, but are always on duty looking for unwary 3CB pilots. Also, ON NO ACCOUNT should a pilot fly off the edges of the map. This, in my book, is cheating.

As SJ says, all of my missions have
- no spawn from air units
- 800m spawn distance
- between 30 and 50 group spawn limit, approximates to 200 to 400 units physically spawned at any one time
- virtualised AI, probably in the range 1000-2000 with about a 1000+ on reinforcement standby