Well. altering the model is (unfortunately) so difficult it is not really worth the trouble. You would first have to get a program that can decompile the model (don't really know of any) and then edit ALL the parts to lower the poly count, then recompile. It would be a major undertaking, and would take up a fair amount of time.
But I have noticed that it isn't so much the poly count that takes away from framerates; it seems to me the higher the quality of the textures, the harder it is to load. If I were going to do this (and I most certainly am NOT

) I would edit the textures to a smaller size and take away the alpha. That may spare your framerates a little.
But if that doesn't work, your best option is to go find AI-specific stuff. Older versions of the aircraft, such as ones made for FS2000 or 2k2 might work better as well. They tend to be a little less comprehensive and more framerate friendly.
Good Luck
Locke