If you look at the SSi thread on here for the aluminum pan, they actually show a detail breakdown schematic of the underside of the legacy showing part numbers... for us 2.5i NA model owners... its just two small plastic pieces in the front right and left corners.
You could probably get your dealer to order you the aero pan from an LGT and put it on... of course there is also the popular SSi aluminum skid plate... it looks really good. 1/8th inch thick brushed aluminum, 20lbs aprox. weight
I'm researching the skid plate/aero pan for my own 05' Legacy 2.5i.. I will probably go with the factory GT plastic pan vs the aluminum pan simply because of the weight... Of course I don't rallye my car, I'm not driving offroad on stone strewn paths, and I don't do highspeed speed bump crossings... I live on the beach in Florida I drive my sube in rain and in the sand.