I just got back from a ~1500 mile trip from Pittsburgh to Alabama to Gatlinburg and back to Pittsburgh. Once on the highway, there is no difference in auto or sport mode that I noticed. They both shifted fine and downshifted when I needed it to. I noticed no difference in gas mileage or anything else. Around town, I like sport mode with the tow haul turned on but on the highway, whatever it is in at the time is good for me. I averaged 21 mpg for the life of the trip. That includes rolling around Cages Cove and going up the mountain to Clingmans dome in Gatlinburg. I am quite happy with the v8, I don't think the v6 would have been much better.
Once the hemi gets to about 80mph on the highway, it really wants to go. That seemed like the sweet spot for mileage and performance on the highway. It really likes that speed or a little bit higher, I noticed no difference in mileage until I started following a older Cobalt SS for a few miles at about 95mph. It dropped down a little bit at that speed.