I doubt that it is legal. In general, anything you do from the cat back is smog legal (but it may not be noise level legal, that is a different issue). Any alterations done before the rear of the last cat must be certified and have a CARB (California Air Resources Board) Executive Order number.
In practical application, the SHO shop Y pipe (with cat, not the off road one without cat) would probably never be questioned since I doubt that many smog inspectors would recognize that it is not a factory item. There would be some risk though.
