It's not only you guys at the Master etc. This AI issue is affecting us also in the Open, Clubman and Pro. I've identified some problems in my DR2 history (only 40 hours)
1. Difficulty level between different categories are too big. That's why you see people complaining about different stages. I have difficulty going from Clubman to Pro. Depends on your driving skills.
2. AI is really fast when it comes to straighter sections. Even though I have full tuned car, with custom setups from GTR Techincal, going full throttle (to my skill level) at 5-6 sections, AI still manages to be lot's faster than me (2-6seconds).
3. AI is sometimes really bad at mid speed sections from 2-4, I can easily go 4-6 seconds faster than AI per section.
4. Sometimes AI makes a "leap": Usually last section of a stage AI just goes "Knight Rider Turbo Mode" and goes 10 seconds faster than me. It does not matter is it 2-4 or 5-6 corners. I'm driving same way than usually.