Sprial -
you're not looking at it right. By running both front AND rear calipers from another car, the bias shift is a function of how those calipers were designed to work with one another and the size of the master cylinder used with them will then only affect pedal travel , feel, and mechanical leverage.
When you run only a different front caliper from another system, you're now changing multiple variables at once. You cannot apply the logic that if the STi Brembo calipers work on the Legacy, then the front WRX Stoptech kit should as well. While it is true your kit is 332 and not the smaller 328, I would suspect the shift rearwards would be more dramatic than the Brembo upgrade, but I have not done the math to prove this. Stoptech HIGHLY advises not running WRX based BBK's on the Legacy.