Of course an intake on a skyline makes no difference in a shop where the temperature is a constant.
The G6 intake isn't one smooth pipe. It has restrictions to make room for other parts and that alone slows things down. A CAI is plenty effective when you go out for a drive, say early morning with cooler air out. You'll not only feel a difference but you'll see it in MPG.
Cooler air is more dense. More oxygen molecules to mix with the atomized fuel. Making a more complete combustion cycle and producing just that much more power. Plus the consistant diameter smooth tubing does quite a bit just to aid in airflow to the engine, which in an N/A setup is worth it at WOT.