A factory stock 6.5 runs a boost pressure low enough to not require intercooling so no meaningful benefit would be gained with an air to air system.
When performance modifications are made that increase boost pressure, intercooling can help to recapture power otherwise lost to the decrease in volumetric efficiency that increase boost causes.
Air to air systems for these trucks do not employ a properly sized grill mounted heat exchanger. Instead, they use a too small unit mounted between the frame rails. The tubing used to route air from the turbo and back to the engine is sized to fit the available space. These two design factors; small heat exchanger and restrictive tubes result in poor intake airflow.
Yes, these systems can lower intake air temp, but they also impose a considerable restriction to intake airflow, offsetting a good portion of any gain.
The type intercooling we like best for the 6.5 is water injection. There is a new water injection intercooling system available that employs solid-state control over multi stage function to provide fuel economy gains in addition to towing performance improvements. And it can be used as a stand alone upgrade with stock level boost pressures.
This system very effectively cools the intake charge and does so without imposing a restriction to intake airflow.