To be honest with you, ram air is worthless at speeds that Xterra is able to achieve, whether safely or not.

The faster you go, the more positive effect you see. You won't see any difference going 70 mph, it's just not enough. Risk of sucking water in is not worth the minor differece you "might" see, but I doubt there will be any.

Also to answer your question, I have not seen a true ram air kit for the X, there's no market for it.