Agree with everything here, I found more of the same.
I've also noticed that they often overdrive the entry to slower corners quite significantly (especially chicanes), making them very easy to pass on corner exit.
After my session tonight, I've become quite disenchanted with the game entirely thanks to the AI experience. I don't know if it's a bug/glitch or a design flaw, but the dramatic speed variations through practice / qualifying / race is incredibly frustrating and not enjoyable in the slightest.
In the GT5 class at Brands Hatch, I was fastest in all practice and qualifying sessions, but then a roadblock in the race. I won the sprint race with ease, without even needing to go defensive, but the entire field was stacked up behind me in a very dramatic fashion. Whats worse is that I think this triggers a "panic" mode in their programming, as the car running in second would consistently make a significant unprovoked error (usually going straight off the road at the final turn), dropping back in the pack to be replaced by the next car in line, who would repeat the exact same situation within a lap or two.
I'm quite comfortable leading a train of cars and holding a race lead, but that wasn't competitive racing, it was the AI being considerably faster than me, but not having the necessary programming to act accordingly. The other big problem here is that if I scale down the AI, I'll be even further in front in qualifying and then they'll be even less likely to be able to race me effectively.
Theres more to it of course, but with how good everything else is, this is such an incredible disappointment. I really hope it gets some attention in the very near future.