I just looked at several different Japanese magazine articles on the GT-R. The numbers look consistent. All the numbers I saw were on the dynopack dyno and it seems to make 480-485 ps and 59-60 kg/m. Hard to interpret the numbers in terms of drivetrain losses... as always. Especially with eddy current dynos since you can calibrate it to a number of different weights.
I think that testing were done on a dynopack because Dynojets are not that popular here and the older dyno dynamics and Bosch dyno have a bit of problems with high power 4WD cars.
I did see a dyno for a rear exhaust which added 26 ps.