The impact that models have is dependent upon three things:
1) Compiler used: FSX native models have a lower impact than FS9 native models (both in terms of AI and static models).
2) Drawcall Batching: I've had to do battle with this recently and it's actually quite a complex subject
3) Levels of Detail (LODs).
The first one is kind of self explanatory, but Nos. 2 and 3 interact in awkward ways and the results of both can vary. In simple terms, drawcall batching takes place when multiple models being drawn have exactly the same material applied,so if you have a number of identical models such as trucks, or buildings - or static aircraft - with exactly the same material, then they'll all be drawn as one drawcall, not however many individual drawcalls you have per model. This makes a scenery vastly better, but it's not perfect. It doesn't work with any model that has an animation block and it's the cause of the irritating problem when models vanish in front of your eyes, because while they're all drawn at once, if the object is too far away to be drawn when the drawcall takes place, FSX doesn't actually draw it. This results in floating windscreens while the rest of a model is invisible, for example.
LODs are the other way of retaining detail while also retaining efficiency. They also have other uses (I use them to make my landing "T"s bigger when you pass them at a distance, for example) but a well LODd model will show a lot of detail when very close and you're only drawing one or two of them, then it'll show less and less detail as you move away. As an example, if you use player aircraft for AI, A2A's models you can have loads of before it becomes a problem. Aeroplane Heaven's, you can't have many at all, because while A2A use LODs, AH don't. Creating them is a huge amount of work, so not having them is far more common than having them!
Personally, my preference would be to use specifically designed AI aircraft where possible, then use a handful of "user" aircraft as AI, then finally use statics. Statics have one use that you cannot do with AI, though. if you want to create a little diorama such as an aircraft being refuelled or rearmed, then statics are the way to go. Create the whole scene as a single MDL file and use SODE, you can even move the little diorama around at random, which is nice. I know my sceneries don't do this... yet. I'm working on it.
Hope that's useful to you.
Cheers,
Ian P.
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