I've noticed, each update with a new track, AI are more racey at THAT track. I've noticed this since the inclusion of Dragon Trail Gardens. The cars used, is also a factor. The faster the cars, from Gr.4 up, the faster the behaviour of the AI.
My Custom Races are 10 minutes, starting from 20th, Boost off, Tyre/Fuel 1x, Slipstream Real, Damage Heavy. Stock setting for car, TCS default, Grid Start, Pro.
I've found AI have put up good fights at 10-12 lap races as well. I've lost to AI numerous times.
Same as TonyJZ, I run N400 vs Gr.4(168,000 Cr. for a win).
AI have NOT been updated at the older tracks. Behaviour is still the same at Bathurst, in particular.
I'm an average racer and I don't divebomb the AI and most of the time, I stay on the racing line. Not so much when AI are brake checking mid-corner. That's how I play. No doubt many players will have different results.