Just one thing to point out. The C5 wasn't an aluminum frame, and neither is the C6, except for the C6 ZO6. And the aluminum frame isn't hydroformed. The only reason they hydroform steel is that it gets them around certain limitations in the ability of steel to keep its strength with multiple bends in a single piece. The aluminum frame is conventionally constructed(for aluminum, that is) using various cast, extruded, machined, and pressed pieces welded and bonded together.
Also, the torsion tube is not a piece of the load-bearing frame. It'* meant to keep the engine and transaxle tied together solidly, so they don't flex relative to each other under torque. It also elimnates the need for fussy and power-robbing U-joints. It does not, however, add any to the structural strength of the car.