Isx detuned?

[MKCgrainhaulers]

I'm sure this has been asked 10,000 times on this forum, but here goes again. My father an I are farmers in the rural midwest. We own a couple trucks that pull 40' hopper bottoms every fall hauling grain to the elevator. We recently purchased a 2004 International 9200i with a Cummins reman isx cm870. Motor has less than 15,000 miles since install. We were anxious to see what the isx could do. "Supposedly" its hp rating is 450 with 1650 torque. We know what 450hp and 1650 ft lbs of torque feels like and this thing falls short. Gearing is good (cant tell you exact ratio) but its turning roughly 1800 rpm at 65 to 70mph. We installed boost and pyro gauges prior to this years hauling season just to keep our eyes on internals. Now, before the bashing begins I'd like to state that I am not on here to turn this thing into some kind of smoking monster. That being said here's the problem. When fully loaded the truck is or should be grossing 80,000 lbs. And on a 2 mile hill, fully loaded, the boost will climb to 32lbs and stop instantly and will climb no more. Also the pyro will climb to 800 and will stop climbing as well. Never getting any hotter. Yes, normally that is a good thing but that's when we really need that 450hp to pull us the rest of the way and it seems to hit a wall and will pull no harder. In other instances such as taking off from dead stop empty or loaded it seems to do the same thing. Power is felt at very low rpms and the same wall is felt. It's almost as if it has the potential but something is holding it back. We are very mechanically inclined. My father has been a mechanic for over 30 years but on mostly no computer controlled diesels when it comes to heavy trucks. One more thing to add. We found out from the dealer that we purchased it from that the truck had spent most of its life used in the Walmart fleet. Do they detune their rigs? Is there somewhere we should look? Or is this the nature of the Cm870 isx? Any input would be very much appreciated. I know that was long and drawn out but I know more info usually helps. Thanks guys