You would have to swap the ring and pinion in order to retain the housing and axles. Therefore it doesn't matter if the donor is LSD. You will just use the SVX LSD
If I recall correctly, you cannot just put the Impreza rear diff in. The axles that go with it are too short.
Can't you just switch the gears in the transmission too? I think someone has done that. That way you retain the extra clutch plates in the SVX tranny that give it its beefiness.