Don't confuse downforce with weight.
Essentially its basic physics - the heavier a car the more momentum momentum = mass x velocity which means you have to apply more force to accelerate the car and more force to decelerate the car.
Friction = mu (co efficient if friction which is a constant for a given surface pair) x R where R is essentially mass.
What does all of this mean? If you want to turn a heavy car then it has lots of momentum which means you need proportionally more friction (equates to more tyre and brake wear) and if you want to accelerate a heavy car then you need proportionally more power.
An F1 car uses downforce to create more R (reaction) while maintaining low weight so it gets the best of both worlds - more friction and less momentum.
The GTR is heavy and it copes, at the expense or brakes and tyres. If it were 500kg lighter with all other things equal it would be a lot quicker.