I think the 5speed auto helps out with the 0-100 acceleration but the 6speed will help with the top speed and fuel efficiency.
plus the ez36d is a bigger displacement so it will technically gain more by those mods in comparison to a turboed 4 with the identical mods. where the Legacy GT gets bigger gains is when you crank the boost up, simply by changing the ECU Tune, exhaust, and intake will not necessarily turn the boost up so the motor will still be getting about the same CFM but would utilize it more efficiently.
its all hearsay, but I love me some Displacement