I get the feeling pace is simulated per position. E.g. say around Bahrain first place is around 1m57.0 and 1m58.5, 2nd is around 1m57.3 and 1m58.8, last place is around 2m02.0 and 2m03.5, etc. regardless of catchup.xml, as I still notice this after deleting catchup.xml.
That's why if Vettel for example pits early and gets stuck in 18th position, he can't go any faster so then he stays in the position as the position dictates his pace rather than his car or his skill.
Another thing I noticed that also points to this is for example when I'm 9th behind say Kubica after the start, for the first 8 laps before anybody pits, he stays consistently about 1.5 to 2 seconds ahead of me. As soon as the Red Bulls, Ferraris and Mclarens pit, before him, and end up behind us, he starts strolling away because he is in 3rd now suddenly. And this is not an illusion, I am in my 6th season now on 40% races and this happens a lot.
That's what I liked about GP4's AI, they were always true to form and related to the specific car. If this game had GP4's AI it would have been the perfect game.