It's unlikely that this path would be appealing to Atari/Eden (ie, it would stop the flow of DLC), because part of the thrill of DLC is that you are being told through the advertising (trailers, YouTube videos, screenshots, etc.) that this car (or map, for other games) is vital to your happiness with the game - that it wouldn't be complete without it. If players had a test drive of the car, they'd realize that it's just a car, and that it's a lot like the 80 others in the game.
If it's a new game mode, this strategy would work. But for car-based DLC, it would be a very poor move for Atari (and hurt us, in turn, by slowing/halting the flow of new content). Admittedly, some players would probably download the pack from the jealousy of seeing other players with the cars, but the majority would realize it's not worth their five bucks and walk away. :(
-Leadfoot