The average mpg is, I think, a rolling 90-day average. As for the wheels, I put some Sport Edition E3's and a 1+ size tire on and saw a decent increase in coasting distance along with an increase in FE. I'm not too sure about the holes in the muffler though. Last year when I did the head work on my other Saturn, the rust exhaust pipe between the manifold and cat broke about 3/4 of the way in two and it just didn't seem to run real well until I had it repaired. Also, there could be a slight chance of CO getting into the cabin. Just some thoughts.
Horsepower is how hard you hit the wall, torque is how much of the wall you take with you.