The added inertia of larger rims can adversely affect your performance, even if they are lighter than stock rims.
While the MOST performance boost (fonrt vs back) would be from putting the stock rims in front, because it is a FWD car, and that is the set of tires that must be turned, vs the backs just "following"... the BEST boost would be all around, because the backs, even though they are only "following" along, they still have mass, and added inertia..
Ray