I've had 2 Cascadias with full-blown 505hp DD15 engines and 13 speeds. Both 2016's. The only difference between them was the final drive, first was 3.42, new one is 3.55.
First one pulled like a hero in any gear but the top two. If it hit a hill, you were dropping two gears and that was that. The one I've got now pulls in every gear and eats hills alive with maybe only hitting the splitter most of the time.
In my experience with the DD15 and climbing hills, you want those RPM's HIGH. I push it to 1700 before shifting on hills. This is a Detroit. You can't drive it like a Cummins.
Sage advice to the Detroit uninitiated; if you can't figure it out, try slamming your hand in the door and driving it like you're trying to get even.