Yes there is at least one guy with Impreza who uses it as a stand alone unit. Thats becuase he has transplanted an EJ25 into an old Impreza, and since he had to rewire the whole car anyway, they did it with TEC alone.
People who are considering using a Haltec standalone, please, go somewhere and look at the wiring harness that comes with it. You will have a heart attack when you see it.