Fuel/tyre sim in combination with timing seems to be pretty accurate, on a 12 lap Turkey race the fastest lap went from being in the high 1.31.XXX to 1.29.6XX, with this year's fastest lap in Turkey being 1.29.704.
AI seems a lot better, I raced them on hard and had to really do my best to keep up and they outwitted me on expert. Codemasters even had to tone down the AI for this preview build, because they were just too quick they told me. They weren't that witty with DRS, but Paul Jeal affirmed that it was down to having too make this build a little easier.
Yes, had several AI cars go off on Turkey, some just for a moment, others crashed. Their cars would stay on the place where they crashed for a few laps.