You do not need to have the injectors tested... If the truck is not running bad ie missing, hard starts, smoke, ect they are fine. Now if the truck was smoking and missing, and you drove it like a retard, yes you can create problems like a melted piston, washed out cyl, ect. Smoke on start-up is normal with a Smarty, exp on higher hp tunes, but an evident haze with stock injectors is not.
Do not pay attention to the overhead, period.
As for your dads getting 21-23hwy... he must drive 50mph on the highway, because that is 750-820 miles per tank...