There are very few pure AI planes out there.  For whatever reason, most CFS2 fliers feel they must be able to fly everything, and as a result even the stock AI planes have been converted to player flyable.

Occasionally you find a bundled download where there is both a player flyable model and an AI plane included in the same package, but even these are rare.  It also strikes me that someone put out some low detail AI bombers for the European Theater, but I can't seem to find them at the moment.

I fly against everything I have in my setup.  However, when looking for planes that you want to use as AI's, look for planes that are multi-lod.  Also some of the older addons for CFS2, those that date back 5 or 6 years and are multi-lod will probable be better AI.  They would tend to have less detail than today's planes and should give you good frame rates.  (The argument against that is that some of the newer planes designed with GMAX are both detailed and frame rate friendly).

I'd recommend flying against different models you already have and note the frame rates then design your missions or focus your Quick Combat on those that don't pull your system down.

This is the Utility you need. Do not Forget you can you cannot have more than 100 AC per Hangar.Its call CFS Quick Combat Editor (now v2.2.0) "Here http://www.simviation.com/cfs2utilities1.htm