I think you're mixing up what we mean by "ST" tires. We don't mean ST=Sport Trac. The tires that came on your trailer are called ST tires, and they're rated for 65 mph.
My brother has an 05 F150 with the old 4.6. It struggled greatly with our 4000# boat. It wheezed and groaned up every hill. His new Ecoboost F150 is night and day difference.
Jus2shy, horsepower does not tell the whole story. While you are correct the ecodiesel does not have as much horsepower as the 4.6, it has significantly more torque. It does not make significant horsepower low in the Rpm range, it makes significant torque. Horsepower is merely a calculated figure between torque and rpm. That's why at 5250rpm, your engine makes exactly as much torque as horsepower.
If horsepower was what really mattered, my 5.7 hem would be a stronger engine than your Cummins since my hemi has more horsepower than the cunmins. It's the torque that matters there.