Greeting! Welcome.
It is not possible.
Although the G25 wheel turn 900 degrees, It's unrealistic for F1 car, But if you notice, while Racing, you will never go Past 360 mark on any circuit. If you watch F1 videos from cockpit view, then play GT5P F1, seems like the same same steer angles are applied.
So yeah it's not possible. GT5 might have an option for this setting in options menu, or GT-25 wheel comes out and has the options on the steering wheel.