If you are out on the highway quite a bit doing 55+ MPH in O.D., then you probably are going to loose a little mileage as the engine is obviously turning a little faster ...
3.31 std = 26 MPG
3.55 opt = 24.3 MPG (@ 7.3% higher RPM)
3.73 opt = 23.1 MPG (@ 12.7% higher RPM)
... will the MPG's drop the same % as the RPM increase as I've listed ?? Doubtfull, but you can probably use the RPM difference as a worst case senerio.
My car so far is an example of the oposite ... the EPA HWY estimate for a base V6 w/ 225 wide rubber is 29 MPG. I've got a heavier Premium model w/ MCA pkg that comes with the wider 235 performance tires (that are std on the GT) and figured I too would take a slight MPG hit. However, I have already gotten 28.8, 29.1, & 30.89 MPG and my commute is only 85% highway. Car is not completly broken in yet, so it's quite possible it may continue getting a little better.
The point is, the EPA numbers appear to be conservative and BEATABLE so maybe you can realistically get a couple MPG better than EPA with the base 3.31's and close to 25-26 HWY with 3.73's in a GT.
Doug