Hi,
For me the best improvement you can have regarding the AI is to enable the slipstream effect. With this enabled it is very hard to overtake a car because you lose downforce when driving behind another car. Even a car which is usually a few seconds slower than you is now very hard to overtake. I think this is very realistic, just remember yesterdays race at Barcelona. I think Nick Heidfeld (BMW) was stuck behind Giancarlo Fissichella (Force India) for a few laps. He just couldnt pass him because of the lost of grip he got because he was so close behind the Force India car (dirty air).
Since I enabled this, I have had races where I got stuck behind a slower car for more than 15 laps! I just couldnt pass them. This is very realistic! Overtaking is now a real challeng (as it should be!).
Hope this helps!