Venturi effect has nothing to do with airflow over a truck. Venturi effect is the increase in fluid speed as it passes from a larger area to a smaller area. Venturi effect happens in a toilet and a funnel.
Regardless, I've seen the truck 1000 times and it always looks level. How can you be so critical over 1/2" (especially with that Hypertech)? 1/2" isn't going to TOUCH aerodynamics.