GT racing has been about bending the rules since the dawn of time.
Look at Mercedes who won the FIA GT championship with the unhomologated CLK GT-R which went from a 7 litre V12 to a twin turbo V8, I think Sauber or Jaguar did the same in Group C, BMW with the V8 M3.
You name it. It is the golden rule, he who has the gold makes the rules. Forget a Skyline in Nations Cup, it will never ever happen. Ross Palmer has cried wolf that many times about what cars will be on the grid.
Also Ross Palmer has said a bazillion times that Nations Cup is not production car racing.
A GT-R with stock turbos and boost would not compete with the other cars, so you would need to increase both, with is increasing the amount of air going into the engine which is a lot like increasing the capacity of the engine which is a lot like stroking an LS1 to 7 litres.
The only companies that design a sports car from the ground up are Porsche and Ferrari. A GT-R is a hotted up Skyline with better engine, better suspension, wider track, improved aerodynamics and styling. An M3 is the same for a 3-series. Sounds a lot like the Monaro to me.
Think of the population compared to Japan or Europe and you will see why they can't make production cars. The HRT 427C would have been great but it didn't happen because the numbers didn't add up. Hopefully a Pontiac GTO 427C comes to life and brings the Monaro with it!
The inability to appreciate anything without lots of cams and valves gives Skyline zealots a lot more in common with V8 knuckle draggers than they might like to admit.