[t vago]

I think, in trying to figure out how much drag is induced inside the engine, we're not seeing the entire point of deactivating cylinders.

The idea is the same as with going to a taller differential gear, in that you want to lower the vacuum inside the intake manifold while reducing pumping losses past the throttle body. Taller diff gears accomplish this by allowing the engine to rev at a lower speed. Cylinder deactivation accomplishes this by physically removing the airflow from 1/2 the cylinders in the engine, and forcing the remaining cylinders to suck in almost twice as much air to develop the same power output to the wheels as before.

In both cases, the throttle plate in the throttle body will have to open up more, reducing the drag loss imparted by the throttle plate to the incoming air. Opening the throttle plate will also cause more air to be sucked into the intake manifold, which, in turn, raises the average pressure inside the intake manifold. More of the pumping work done by the engine goes to sucking in air/fuel mixture, and less goes to developing a vacuum.