Supercar all one word means a high performance usually exotic from the factory, a Super car is a car that is good at what it does.
A Rolls Royce Phantom is a luxury car, not a supercar, it's for touring or being driven around in, a Supercar is a car that only those who are lucky enough to do well in life can have and enjoy, it's the cars they have dreamed about owning since they were a kid, they are the posters you had on your wall when you were very young (not after watching Fast and the Furious), they are the cars that when you see them you go "Wow" and your mouth stays open. They are the cars that epitomise how far you have come in life and bring a smile to your face and those around you. A standard Skyline does not do that, a Ferrari F40, a McLaren F1, a Lamborghini Countach, a Porsche Carrera GT and hell even a Saleen S7 does all of this and those who own these cars can only know the thrill and excitement of the time you turn that key.
A highly tuned sportscar might put a smile on your face and might be faster than a Supercar but at the end of the day you will look at a Supercar that pulls up beside you more than anything. And if you were to park a Ferrari F40 and a Skyline Z-tune on the street together, no prizes for guessing what would have the biggest crowd.
There isn't as far I can think a proper Supercar from Japan (whatever happened to that one shaped like a bullet?), the NSX is a very competent sportscar but it's not a Supercar in the real sense of the word, on second thoughts the NSX is a Japanese Supercar, it had the styling, the pace and the non-practicality, it handles very well but is not as poised and requires attention to drive hard so yeah I guess the NSX is a Japanese Supercar after all.
Due to films like Fast and the Furious the term Supercar is being associated with cars that don't deserve the title.
Again I say a lot of cars are super cars but they are not Supercars!
As Jeremy Clarkson said on one of his earlier Videos (wow that old huh?)
:D