Unfortunately you are 100% correct. Most (If not all) cars in Test Drive Unlimited are so unrealistic that it must be a joke of some kind (Same goes to latest NFS's [Most Wanted, Carbon, Pro Street]). I guess the reason you can't see any Porsches in Test Drive is that they didn't wanted to include their vehicles to that kind of arcadish game (Even someone says that Test Drive is simulator. What aspect it simulates? Driving fast, okay. But the simulator ends when you hit something/someone, go to a turn or even brake). There are some nostalgic cars in Test Drive, and I started to laugh when I drove 60's car over 170kmph. Don't remember how fast did I drove Jaguar E-type, but man... I assure you it accelerated faster than real E-type [I know]. Heck, it accelerated even faster than nowadays "average sized sedan".
Shortly put: All cars behaves unrealistically, but I guess that is ok for the manufacturers. They are fast and easy to drive. Oh, and top models gets onboard as long as you have expensive car.
From topic to off-topic to angst. Sorry about that.