Joe, I agree with mike about not setting tile on plywood, especially in an area that will probably get wetted from time to time. The thing is, the whole floor structure needs to be considered. It isn't clear to me whether the sub-floor is existing, or you were planning to install it. If the latter, what is under it, and why do you want to do that? If the former, be aware that cement bd. requires at least 5/8" (preferable 3/4") exterior plywood over joists (or sleepers) no more than 16" oc. and must be bedded in mortar.