Ford 6.7L 1050 ft lbs TQ at 1,600 rpm, 475 HP at 2,800 rpm

4x4ord wrote:
If our C15 Cat makes 550 horsepower with about 40 psi of boost at 1400 rpm it would seem likely that a 6.7 spinning 2800 rpm would require similar boost to have similar airflow per hp if it were making 475 hp. Is there some reason to think that 38-40 psi should be unobtainable out of a stock 6.7 turbo?

That depends on the efficiency of the turbo which is effected by many things. I don't have a compressor map of a the new PSD turbo to be able say yes or no. It may be able to make 40 psi of boost, but it may be over spinning to do it. Just because two turbos are able to create 40 psi of boost, does not mean they have the same amount of air flow at 40 psi. There is a point where the blade of the turbo is spinning too fast and it is chomping through the air which reduces air flow. A larger turbo will generally have more air flow at 40 psi than a smaller one at 40 psi.

There is also the CFM of the exhaust as well. The big trucks have much larger exhaust and emissions equipment allowing for less restrictions in exhaust flow.