What are the sizes of the pistons in the calipers you're using? I'll be able to tell you whether or not you're in the right ballpark in terms of brake torque output when mated to that 317mm disc. What you don't want is to massively over-amplify the front brake torque, and foul up the brake bias relative to the rear. In that situation you'd actually be hurting the car's braking performance (lengthening stopping distances, potentially interfering with proper ABS function, etc.). You'd have more of a heat sink and lower overall weight, but that doesn't do you much good if the car doesn't actually stop as well!
I'm not trying to discourage you or anyone else from doing this swap. I fully appreciate your financial situation and your ingenuity in working towards an alternative solution. But before everyone runs out and starts buying parts, they should make sure it's something that can be done safely and properly, not just something that can be done.
I checked on the Legacy GT disc you're using...it weighs 22 lbs., +5 lbs. over the OEM front FT86 discs. I actually owned a 2005 Legacy GT 2.5 Turbo wagon (5-spd), and even had a pair of these discs sitting in my garage.
