My very brief and unscientific testing on the way to work this morning has led me to believe that Sport Mode may also reduce DFM shutting down cylinders. Driving on fairly flat highway at constant speed and switching from Tour to Sport, the instant MPG dropped when going into Sport. Couldn't feel any downshift and I didn't think to look at the RPMs as I was coming up on traffic, so more testing is needed...
I have the GM performance exhaust, and it 'seems' like it is louder in Sport, which could be the engine running on more cylinders more of the time, but again, more testing needed.