After looking through the GT Pro race grids, I noticed that the cars were limited to Astons (lots of Astons), Corvette C6R's, Audi's, Listers and Koenigs. No Saleens, no Apollo's, no Vipers. Are these cars restricted, or just not popular? I know the Viper is a handful, but the Apollo and Saleens are lovely to drive. What is the thinking behind the car choices I wonder?