15" rally wheels may fit, but you'll have to go to various sites to look it up. Team dynamics motorsport's site has the set i'm thinking of. It says 'not for road use'.

15" Subaru wheels will NOT fit. Aussiedan's been running GD WRX brakes on his legacy for a long time, and my Legacy L 15's wouldn't fit over them.
15" steel wheels will NOT fit to my knowledge. Even some after market alloy 16's required some grinding of the caliper to fit correctly (Gabe's old Legacy L project with WRX front brakes and Rota D2's).