600pp Expert race Challenge done with Toyota TS030. Ran a 24 opening lap with the rest in the '23's except for my final turn mishap I would have had a 8 second margin minimum.
Good luck, you can do it.
So, what did they do? Turn the AI up yo 11, maximize the pp probabilities and find just the right car? I am not sure, but I have to say there is nothing wrong with a good challenge.
Also, I forgot to mention you only need to use 2, 3 and 4th gears since the trans defaults. You can of course play with top speed setting and individual gears if you chose, though I did not bother with that here.