I don't recall doing much with the tuning. For the circuit events, the Lambo LP640 is fastest, but that's a DLC car, and if you don't already have a save with that car, you can't get it right now because Origin server is not sending unlock messages to the game to unlock it. The Apollo is probably the next best car. However, on high speed tracks, both of these cars have a downforce issue that pushes the car body into the pavement and lifts the rear tires, resulting in snap oversteer. It is a bug in the game that was never fixed. This is one of the reasons I prefer Shift 1, although it has a bouncing bug on the banked oval tracks.
I made a video with the Speed Hunters Challenger, 10.145 (link is below). The Speed Hunters Viper is faster, but the AI will be faster as well, but you don't have to win and set the 10.50 time milestone in the same set of runs. If you have it, the Lambo LP640 is fastest and also easy to drive in drag mode, and can run under 8.5 seconds.
I'm using an analog controller, actually two joysticks that appear to be a single device to games, left stick is throttle + brake, right stick is steer, similar to the old style radio control transmitters for cars (also for model planes). Shift 1 is better when using an analog controller, but Shift 2 seems designed for digital gamepads. I had to borrow a digital gamepad to get through some of the Shift 2 drift events, as some of them are virtually impossible with an analog controller.
https://youtu.be/wJ41DbFzQyQ