I'm not seeing it....even with jacked fuel pressure and the injectors on the edge of going sonic. That X303 cam won't support anywhere near those power levels with those cubes and that top end.
Now....if his claims were closer to 400hp, I'd buy into it....but not 500hp. I think he's confusing what his engine internals are rated for and what his engine will actually make with one and other.
Still a sexy car though