The pre '14 5.3 don't compare with the newest gen 5.3 engine. With 355 hp and 383 torque it will out perform the older version. Low end power is amazing for a smallblock engine.
My wife has a '16 1500 crew cab 4wd chevy 5.3 355/383 engine 6L80e 3.42 gears. This truck replaced her old '06 same 1500 crew cab 4wd 5.3 4L60e 3.73 gear chevy truck. Apples vs oranges.
I use it sometimes pulling a 12k gvwr flatbed with a 12' tall tractor/cab with around 9460 lbs on the trailers axles and 1020 lbs on the trucks rear axle. Of course the tractor can be placed anywhere on the trailer for balanced axle loading.
We stay within 250 miles or so on jobs so it gets some leg work on the road.
The '16 5.3 6L80 is hands down better towing a load than older 5.3 versions. In the hills I run the 6 speed in 4th and 5th on the flats. Empty or towing I see no mpg differences between 5th and 6th.
Of course the 5.3 won't compare with my Cummins or the 3.5 EB engine in the F150 but IMO it does good for what it is.
For more 1500 towing power the 6.2 at 420 hp and 460 torque NHT tow package with the 3.73 axle ratio is a butz kicker or 3.5 EB eater says some owners like to brag.