Well much of this depends on the test and equipment used. Here is a comparison between the GT-R and 997TT.

http://www.autoblog.com/2008/03/13/nissan-gt-r-meets-the-dyno-997-porsche-turbo-and-e92-bmw-m3/2

Whatever the real power, isn't the sole issue performance? Assume for the moment the GT-R has more hp than advertised. So what? It is faster and about 40% less expensive than the TT. Or does that matter?