The IS's had Eagle GA's on them... They came standard on the '96 Chrysler Concorde and LHS, the worst tire I've ever experienced, they give next no feedback/communication to the driver. Lexus probably bought them to lower the costs of holding these events.
I'm sure the IS300 would perform way better with the bridgestone RE040's. The Bridgestone RE940 all-season tires gives out 3 times as much road feel as the GoodYear GA's IMO, who knows how well the RE040's will perform. Looks like you'll still have to wait to test drive a IS from a dealership to really feel how the car handles.