You will have to become a virtual mechanic if you want to win at the higher levels. And because each car handles differently, you need to make sure your new car suits your driving habits. Some high performance cars can easily spin out while others have incredible control. You will find yourself buying and selling cars to find the stable of machines that fit you. The AI in Gran Turismo is second to none.
Opponents are aggressive without cheating.
You will sometime see your opponents off in the grass. But the opponents are very good drivers. They hold their lines and take advantage of your mistakes. The AI is consistent across both the Simulation and Arcade modes. It is here that the game breaks down.