Before you go further, have you considered picking up a MT Legacy, say a 2009 6MT Spec.B? The reason I ask is that although what you ask for is doable, the options of strengthening the AT are much more limited than what you can get with a MT.
You gotta think about the rest of the drivetrain too. Rear and front diff. You can do all sorts of swaps on a MT, not so much on AT.