Subframe connectors

Ken,
The roll cage consideration certainly makes sense. But, when you talk about stiffness (i.e., the ability to resist twisting and bending) of a tube the wall thickness has a minimal impact compared to changes in diameter.

Wall thickness becomes imoprtant when the need to resist impact forces are important. Bicycle tubing is a good example, some of the real high end bikes are extremely stiff for efficient pedaling but the tubing wall thickness is so thin that you can dent it by squeezing it with your fingers. Once it's dented it's looses it stiffness dramtically. A roll cage by definition is subjected to impact forces and that's why the wall thickness is so high.