Yes a gearing change would lower MPGs for those cars equipped with it, and I guess would lower the overall average MPG of all cars if they are somehow keeping track of how many are sold with higher gearing versus those with lower.
BUT they can still advertise the 29 MPG highway as they are now, just as ford advertises 31 for the mustang. We all know you can only get that with the higher geared mustang, but general public won't realize it. I'm not saying lower the gearing and thats it, but instead give us another option just like ford does.
But again, as I said before, I'm sure offering a gearing change could cost GM more money in development and assembly and such rather than doing something simple like writing a new tune or adding intake/exhaust tweaks.
5th gen IS SOLD! 6th gen built 10/14!!! Can't wait!!