4.6 V8 the diesel is a slug by comparison.
It runs ok in the normal mode, but there are some hills round here where it labours too low in the revs, then has to drop 2 gears instead of one just to maintain the speed limit. Or you have to provoke it with a large throttle input, but then hard to not exceed the speed limit. In sport mode you only need to flex your toe and it'll kick down 1 gear instead of two, but then allow you to easily stick at the speed limit. In this situation I'm sure sport would be more fuel efficient, not to mention just nicer to drive.
In general sport mode allows you to drive it on the throttle. Lift off and it'll upshift, so you don't need to see higher rpm than normal mode. But the entire vehicle is far more responsive for part throttle applications. Flat out wide open throttle, sport and normal mode are the same, they both run to the red line. It's the part throttle response that is so much more improved, letting it be in the right gear more of the time.