although there doesn't appear to be anything added on (ei missing deliver missions, avatar selection/customization) I can confirm that the retail version of Test Drive fixes everything that is wrong with the beta verison, and I mean everything. Clipping, draw distance, frame rate, car handling, and, best of all as far as I'm concerned, the phsyics of Enemy AI cars. You can officially push Enemy AI cars off the road if they get in your way, a feat that you could have never accomplished in the beta version of the game, because in the beta phsyics simply did not apply to AI cars.
This is the main reason why I could win any races on normal mode while playing the beta. If an AI car touched you at all, especially in you were trying to turn, you'd spin out every time, while you could never force them off the road or away from their path, no matter how hard you tried. They were completely undestructable while I, on the other hand, was extremely destructable.
In other words, the game was frustratingly unbalanced before but it is unbalanced no more.