There are limits to how much additional power and torque any drivetrain can handle. We know that the GM Duramax powered trucks have about 75hp or a little more cushion in the Allison before slippage can occur - depending on how the truck is used.

My experience has shown that a little added power can improve fuel economy while towing, because the transmission can remain in OD more of the time, and not downshift when approaching a minor incline, like a highway overpass.