Depending on the setup you could gain tenths of a decibel on the meter, but chances are your ears couldn't hear the difference. Usually a larger box is going to be more efficient that a smaller box given the same power level. So by flipping the sub around you are gaining the amount of airspace that the sub displaced. However this amount is very small, typically .10-.15cu/ft.
You also have to consider impedance rise and different box volumes will give the sub a different impedance curve. So the only real way to tell is by doing lots of testing and 99% of people out there are either too lazy or they don't have the necessary testing equipment or know how.