Other than the Amsoil 5w20 oil, the honda is bone stock. In fact, it's even got winter tires on it. Since it isn't broke, I don't have any plans to mod it.
Yea speed does seem to kill the mileage. Although we had made a rushed trip out of town a few weeks ago where we pushed the speed limits up to 100 MPH and had done a whole bunch of heavy acceleration. The mileage dropped from the standard 38 mpg on the highway to 32 mpg. It wasn't nearly as bad as I had thought. Pretty decent mileage for a mid-sized car.