Your the organizer, you have to set the regulations. Some cars have 10/20 downforce, others have 35/60 downforce. Because different players interpret the rules differently, your going to have differences in opinion, whats ok, whats not etc. You are not making the car regulations clear enough. Each team will test to what they think is the regulations, rather than testing to what IS the regulations.
I know there is a lot of depth and strategy to an endurance race. As I said, I help prepare and run in an endurance series and have been most of the year (only 1.5hour races though, not 24hour) Regardless, in GT5 without engine failure, mechanical breakdown etc, it's more about going fast, and a slow car from the get go is a slow car at the end of the race. Wouldn't it be better for all cars to be as close together on pace as possible from the start and let the drivers battle it out for the win? That's more of a race than 'I'm in a slow car, I hope the fast guys crash so the end of 24hours I'm not last'