From what I understand the Alpine S/C is still CARB pending.

You should search on this question, because it's been asked before. But to change the Xterra NA engine to a S/C Xterra engine would take alot of work (not only engine internals). Your answer to the question is this "it'd be cheaper to buy an Xterra with a S/C than put a S/C on an Xterra" (that's in regards to the stock supercharger. Unless Alpine did something than nothing has changed)