Downshift or use the OD lockout when hitting steep grades. That's what it's for.

There's several reasons to drop a gear, all center around heat. Heat that damages the trans and engine.

At lower engine speeds it takes more boost and fueling to create the same power, this drives up EGTs and can hurtthe turbo and possibly burn valves. 2300 rpm is a cake walk for theCummins and better than lugging at 1800. If you have egt readout try to stya below 1300.

At lower rpm the torque conveter is being worked hard and building lots of heat. Higher rpm puts the converter closer to it's optimum speed creating less heat buildup.

As for loss of power, when dropping a gear you do have to add more throttle. The engine is spinning faster and moving more air, you need to add fuel to keep up.

Maybe this is something that you can tune with the programmer?