If you have surround sound, set sound to theater range and crank up the volume to realistic levels, just about all cars sound completely real. There's no way to complain about it in that situation.
I think that most games edit their sounds so that they sound good at lower volumes, but this may affect the integrity of them at full volume. The trouble is that we lower GT5's volume because it would be impractical to have the sound of a racecar at full blast in a house full of family, so it's like we're listening to the race with earplugs. I think that the sound in Gran Turismo 5 is among most realistic of any game, but most of us have to keep it fairly quiet and so can't hear all the fine little details.