EB currently makes 380hp/460lb-ft in Navigator trim, which is more than the 3V V10 we can't get in E-450 Class C guise.
Is the Eco-boost the limiter on GCWR, or is it the transmission/vehicle they're installed in that's limiting GCWR? I don't see Ford ever allowing 20k+ GCWR with the F-150 or Transit.
Given sufficient cooling, I don't think there will be a huge difference between the two. As for GCWR, since none of the V10 Class Cs I've seen were rated for more than 5k lbs of towing and 14,500lbs GVWR, there's not much of a jump from 17,100 (F-150 EB GCWR) and 19,500lbs.