Well, the only car I tried so far is the car you win in the Nurnbringring 24 hour. I may have change the tramsmission for top speed, but I think I kept all settings default excelt I know I decreased the front and rear spoiner by 5 each. This allows the car to go just slightly faster than the competion on a setting of 3. The competition was pitting every 8 or 9 laps, and I was pitting every 5. I also may have added about 30KG ballast, I remember I experimented. If you are using B-Spec, start with 4-Fast/Overtake and change to 2 or 3 before you come to the end of the long straits. Once you are the lead car, you should be ably to keep the setting to 3 and not worry about flying off the track. I used 2 for Sarth I and 3 for Sarth II. I think the cars to beat that I had were the Nissan R89C in Sarth I doing about 3'29 per lap and the Sauber Mercedes in Sarth II doing about 3'11 per lap. At least that's what my notes reflect.
On the other thread, most people are having issues coming off the track. I believe it is due to using 4 or 5 with the way the high speed draft can spin the cars out of control when passing. It could also be the loss of HP all the other cars have over distance and/or the body loosing it's rigity over the roughness of the road. The more times you lap a car, the higher the risk of the drafing throwing you. There are 3 places on the straits that the tires completely lose contact to the road. If the AI is half in the draft, passing, when off the road, the car will spin!
Either way, I have not experienced the troubles other mention, and I believe it is because I set my speed to 3, not being too agressive, and the fact the Formula GT does not suffer HP loss, or any other damage from the 'special' nature of it.