I know this is an old thread, i was just searching and found it but it hasn't seemed to have ended with much factual information.
In Gran Turismo, the steering lock on your wheel is set to 900 degrees for all cars, however, the front wheel turning angle is set correctly to correspond with the steering wheel lock. The F1 cars feel very good and drivable to me after the spec 2 update. Just because the steering wheel turns a full 900° doesn't mean that the steering ratio is increased in the F1's.
In real F1 cars the steering ratio is fully adjustable within a certain range, meaning you can adjust how much the steering wheel turns and how much the front wheels turn, some drivers like Fernando Alonso and his team mate Massa both prefer a higher ratio (more steering). This is somewhat possible in GT5 by adjusting the steering sensitivity within the "Driving Options" in the track menu.
Here's a video of me with the T500 with F1 wheel doing a seasonal event at the Nurburgring 24H layout. I have all the driving aids turned off, this includes the ABS at 0, the steering sensitivity is at 0 as well and i actually prefer it this way because it allows for more precision, it feels like the front wheel angle is about 14/15° at 390/400° steering lock, which is realistic. If you increase steering sensitivity, it increases the front wheel angle while maintaining the same steering wheel lock which shortens the ratio, same effect as in real life:
Also, as a visual reference, look at this video of Fernando at the Ferrari test track in the F10: