[gbb357]

Originally Posted by Autoblog

Eric Hsu, one of the many men behind XS Engineering and also a veteran of A'PEXi and Cosworth, got a hold of the February 2008 issue of Japan's Option magazine in which the pub strapped the MINE'S-modified Nissan GT-R to a Dynapack and ripped off a couple of runs. The GT-R was equipped with the prototype catback exhaust we featured a couple of weeks back, and although the design is far from streetable in its current form, it still created just over 20 hp over the stock setup by reducing backpressure and allowing a 1.42 psi increase in boost. Although Hsu maintains that the Dynapack might skew the results by up to five-percent, the 507.9 ps (500.95 hp) rating at the flywheel is still impressive considering that nothing was done to the GT-R's ECU.

[Source: JDM-Insider]

http://jdm-insider.com/Blogs/Eric/?p=540

The dyno test was taken at the flywheel, so If this is the case, then the 480hp rating that Nissan is giving seems to be accurate.