Ok, I can spell it out even more if you'd like. You said the V8 is a waste of gas. In fact it goes 4 miles further for every gallon of gas burned on the highway. Now it's about how many wheels are driven? I guess everyone knows AWD uses more gas, but no one (or at least you don't) knows that automatics use more gas, as do bigger cars. You made the claim about mileage and were wrong, it's as simple as that.
Apparently I should spell out the torque too. 323 is greater than 300. Specifically, it is 23 lb-ft greater. While it may be "close to the same" depending on your point of view, there is no circumstance under which 300 is more than 323. If that's not quite clear, then let's pretend this "T" is a pound-foot. Then here's how many more the Impala SS's lame wasteful motor has over the Subie:
T T T T T T T T T T T T T T T T T T T T T T T
As to maintenance, "everyone knows" that turbo cars typically have more frequent oil change requirements, tend to go through plugs faster, and in general just do not last as long. The turbo itself will likely not last the life of the car, unless you consider the car's life to end whenever the engine/turbo lets go.