pound for pound a c-section can be made stronger than a boxed beam, just marketing hype

 

Nope... dig out your physics book and study mass moments of inertia. (a.k.a I^4 values.)

There's a reason drive shafts are big hollow tubes, flagpoles are big hollow tubes, road sign posts and many other structural members are too.
These types of structures not only resist basic deflection or "cantilevered loading" on the axis of the member, but offer superior resistance to torsion - a characteristic often experienced in vehicle frames, and being demonstrated in the example posted above.

In short, you can have an I, C, or boxed-shaped beam that will all deflect equally under axial loading, but the box will resist torsional loading (twisting about it's axis) better than the I or C- shapes. I'll hand you economics - as I's and C's can be rolled to final shape in mass, where boxing requires DOM extrusion or welding - both of which add cost and complexity. Which is what I said it came down to anyways... Toyota saved a buck where Ford and GM have stepped-up.

You can start reading here... http://en.wikipedia.org/wiki/Hollow_structural_section
"HSS (Hollow Structural Steel) can also be used as beams, although wide flange or I-beam shapes are in many cases a more efficient structural shape for this application. However, the HSS has superior resistance to lateral torsional buckling."