The lip on that Legacy, my Legacy and Jon's Legacy put out what, 1 lb of downward force at 100mph? These "lips" are usless for creating downforce and do not smooth out the air traveling underneath the car. They are more for show than performance.

Now if you had an adjustable front splitter, that was low to the ground (as in an inch or less off the ground with a set of canards on your front bumper and the whole underside of the car covered in smooth panels and a big diffuser in the rear, that would be track effective.