GCWR of the 3.5L EcoBoost in the F150 is 17,100 pounds, and in any test performed at full load it easily outperforms its competition. Even the 2.7L EcoBoost handily beats the 3.0 diesel in the Ram1500, pulling trailers in the mountains.
Max GCWR of the EcoBoost Transit is 13,000 pounds, vs 13,500 pound for the EcoBoost Transit. The diesel adds 325 pounds to the curb weight, which means it tows a whopping 175 pounds more than EcoBoost.
The Transit EcoBoost was tested under Ford's medium duty test cycle, same as the diesel in the Transit or even the 6.7L diesel.
I don't know why you even posted the Iveco's GCWR. It isn't sold here, therefore doesn't have a GCWR for North American applications. Most global Ford's also have higher GCWRs outside North America.