If you get the right offset & roll the arches you will get away with running 18's alright.
BUT - from experience it makes the ride far too harsh & takes away a considerable amount of the handling ability which I assume is the whole reason you bought a Subaru in the first place.
Not worth it - get a nice set of 17's & buy a decent set of lowering springs & you'll achieve nearly as nice a look but without the issues above.