not true. the gtr has always been and always will be a track monster. the issue here is that the focus is on quartermile in the media. nissan also has to improve there launch control with the competition, ie porsche, so it must match there performance as close as possible.
also note that there have not been many changes to the gtr since its release. very small changes have been made.
I am also seeing the future lol, nissan will release a 580hp gtr in the future. the FIA GT3 GT-R is just below 600hp and also one of the creators of the engine from cosworth said on an engine dyno its capable of running reliably at 600bhp. with this information i predict the vr38 will go into a transformation in the future. maybe it will be a nismo tune engine - spec Z or something of that sorts. the final evolution of the R35.
With that said, its unknown for sure, but to come back to your suggestion of peroformance on a race track... it will more than likly be increased. currently Mizuno has said there budget for upgrades has been slim but he has taken the best upgrade path's with his limit on spending, and i would agree. seeing that the gt-r will now be around for a few more years and Mizuno has HINT'ed at a few other things in the project's future, its going to be very interesting to say the least. NDA's and trusting people are keeping a tight lip on what they already know.