I agree with AJP. Also, not sure how the mustang handles it, but in my SHO, if you are in S, it will stay in 5th gear at highway speeds (85-90 mph). I have never noticed/gone fast enough in S on a normal drive to get into 6th gear. On the race track I don't think I have even gotten into 5th (drag strip) so no idea of the conditions needed for 6th gear in S.
This will hurt fuel economy on a road trip especially, as it will hold you at higher RPM than the speed requires. Part of the exhaust note difference in sport mode driving is due to shifting later/holding higher RPM levels for the same speeds (due to using a lower gear)