Hey..
Just a note after having my last car all kitted up i would say NO..
Plus if your keeping the front bumper OEM i would say no aswell. Trust me leaving the front as is and putting bigger bulkier side skirts on will look a little out of place. And a rear lip should really be moulded in the make one but i think it will unbalance the look of the car..
The sti is a sweet looking ride as is??? Lower it a bit more and i think it would look sweet as..
After saying that at the end of the day its your call, Its your car do what you want to it..