Just attempted to build and tune this car. Fitting the adjustable Forza front aero increases the speed stat, reduces the handling stat, and is permanently set at 0. It cannot be adjusted.
I suppose you could make the argument that it’s for realism. I doubt putting an air dam on a 5,000 lb. shipping container would provide much downforce.
Are there any other front bumper options to try? I seem to recall running into one instance where the Sport and Race options were inverted.