AI will be a FPS-killer no matter what.
The only thing is that when you disable the default fake airlines, and install just a few real-world AI companies, then you end up with much less traffic, which means much more FPS

The freeware AI packages are, unfortunately, all made with FS9-format aircrafts, may it be World of AI or Alpha India or other packages I'm not aware of. That means they will not work in "DirectX10 preview" mode in FSX. They work perfectly in the normal DirectX9 mode, though.
Only some payware AI packages are containing FSX-format aircrafts.
I went for the freeware ones. I installed some of the major airlines for the main countries I use to explore:
- northern America+Canada+Alaska,
- Western Europe, mainly France, Spain, Italy, Germany and Switzerland,
- Eastern Asia, including China, Japan, Korea and the Himalaya zone,
- Northern Europe, mainly Sweden and Norway.
The performance remains quite nice, although I use to stay away from the big major airports. All other places, from "normal big" to small airports, have good performance, and I can even get some relatively crowded skies at times. Only thing to keep in mind with FS9 traffic is that it comes with FS9-format flight-plans. FSX can read FS9-format flightplans of course, but there is a bug that disables all of the other FSX-format flightplans, for planes and boats/ferries. Using the freeware AI Flight Planner (AIFP), you can easilly spot those FS9 flightplans and convert them to FSX format to get your full traffic back
