Hi CG. I find the most noticeable difference is the firmer suspension rebound control, and the transmission shift point mapping/how the trans holds gears at higher RPM's. To a lesser extent, I do notice the slightly firmer/stiffer steering feel.
The video that BOT_ROCKET provided that shows a Fusion sport going over the speed bump really illustrates the suspension rebound control very well, in my opinion.
After experimenting with the various settings in my MKZ, I now have S set up for the firmer Sport Suspension settings, but the Normal Performance settings. Only because in my particular case, I sometimes like the firmer suspension rebound control, but don't particularly need the more aggressive trans mapping. I guess I prefer to save a little gas.
If I did a scientific sample, I would say that the majority of the time I leave the trans in D, using S once in a while.
I realize the Fusion Sport does not have all those settings from the factory, but they can be added with FORScan in most cases.