the 545 is garbage.. it was a miserable train wreck in its first gen form and gave the truck a bad name in the 3rd gens. Additional, stock or not that thing hunts for gears like its going out of style. also, it does not downshift sequentially and holds gears for too long so if you roll up a hill you're forced to goose it before it falls on its face going 20mph in 4th gear before it kicks down to second - junk, and makes the truck drive like complete **** and paired with MDS its even worse.
As for holding power.. yeah sure, i guess it will get the job done but the trucks power and tow rating are so close to the max it can handle its just scary to know how fast things can go wrong if it wants to let go.
the ZF is 100 times better, you even said so even tho your saying your not saying so

(read your last sentence again

). Compare the issues people have had with the ZF's with the 545's and you will see its built to perform, remember they put this exact same transmission in the Scatpacks wth 392's and tune them to shift hard as fu*k from the factory, and I have not heard people blowing them up like we have horror stories of the 545's.
I had a 2014 with a 545, and a 2014 with a ZF, both tuned, both stock. they are COMPLETELY different trucks because of that trans. If I were to classify a perfect trans for this truck, it would be the ZF.