All the Gran turismo games have been quite understeery feeling. I never realised how much until i had got Rfactor, played that for a bit and then went back to gran turismo, to realise that i just couldn't drive the cars anymore due to the huge understeer.
That said i love Gran turismo games, and played them fine for years, having got used to the physics with nothing else at the same quality to compare it to until recently, but i do hope that they fix it for GT5. Its hard to fix the issue with setup without screwing up the car in other ways, since the physics are naturally that way, rather than it being a car to car setup thing.