After investigating the price from Route-KS. the price is actually not bad.
As for the performance differences. Two are really the same, perhaps little more down force with the GT.
I don't see the value of $450k for the car when the only differences are different shapped CF F/R Bumpers, Mugen Snorkel and hatch ($8000), and side AIS Pods. If you look at the weight spec, it is same as the regular NSX-R of 1270kg.
However, since no one had the chance to test one (only five produced), there is a great secretcy withi this car. Unless we get to see detail photos, the rumor of CF chassis is just a rumor.