I just noticed this week that if I use the auto open on the sunroof it does not open all the way. It stops about 1-2 inches shy of completely open. Another tap of the open button will open it all the way.

Is this common on all IS's?

It's normal. It is designed that way because when open all the way it may cause more wind turbulence than at the first "touch".

^Correct; the 1-touch is for the optimal opening with least wind-noise.