No real answer to this question ultimately, it's one of the chicken vs egg arguments of racing.
The car changes what's possible, but you still have to drive it. A monkey could probably win Le Mans with an X2011 vs the GT5 field, but a good driver in a stock Corvette can beat a moron in a 908 at the Ring.
In GT5 it's much less meaningful than reality because in GT5 you just need to meet a given pace. If you can do that pace with a slower car, you'll win. Of course a faster car will make you faster. In reality it's a competition between [ideally] quality competitors in comparable equipment. A faster/more reliable car helps, but again you need a team of drivers to go fast enough without making mistakes, and a pit crew doing the same, and somebody has to find the optiumum setup balance.
I think it goes something like this:
If the other guy wins, it's his car.
If I win, it's my driving.
If the other guy blows it, it's his driving.
If I blow it, it's my car.