I wonder what your engine RPM difference is at highway speed...
Tire Rack says 635 revolutions per mile for your 285/70-17 Duratracs, versus 683 revolutions per mile for Dunlop Grandtreks in 245/75-16.
I agree that you'll feel a difference in power and acceleration due to the lighter weight, smaller diameter tires. But won't the extra RPM required to hit 60-65-70mph on the highway offset that?
Let us know when you hit the road. Be careful and good luck! :safari-rig: