Stock Legacy GT ( pre-'04, anyway) wheels are 16x6.5 with a 53mm offset.
The aftermarket is rife with wheels in a +48mm offset. As I'm guessing that had been your plan anyway (you already have the steelies for snow tires), I say go for it. Besides, unless you buy used (check out the Forester Forum near the bottom of the Forums list page), aftermarket will be cheaper than stock from the dealer.
Lil' Red '98 2.5 GT Wagon - Pics