Remember GT3 and GT4 when you reached 100% completion?
In GT3, you were awarded the F094/S (a replica of the same car that Ayrton Senna was killed in). This easily was one of the best (if not, the best) cars in the whole game. Although you could also win the car in another way, it was still fairly difficult to acquire.
In GT4, you were awarded an all-black Formula GT. It was a one-of-a-kind color scheme for the car. Completely different from the standard colors. Now THAT was rare. And it completed the collection of all-black, special edition racers also found in the used dealership every 694-700 day cycle in the game.
Assuming of course, that something is done with GT5 in the near future, we could get something like a one-off X2010 (I know, lame and totally unoriginal) in a special color scheme for 100% completion, or something that sets it apart from the rest (it has to be unrelated performance-wise, to make it even a slight bit fair). Except that this particular X2010 you win is ID-tagged/locked to your PS3, and only your PS3. Kind of like the Chrome Line and Stealth cars from the special editions of the game. So no duping or trading is allowed with this car. That's your legit proof there that you've conquered GT5 👍