I know dealing with 8 foot sections of 2x3 boxed tubing building motorcycle trike frames we found little difference in flex between 0.120 and 0.095 and 0.083 when tied in with the other frame parts all diagonally braced. And the diag's were all 0.063" DOM round.
But we are talking a 800 pound trike and not a 3200 pound car. But then again the SFC's are not real structural in most use. I bet you could get about the same effect with a stick of 1 1/2" 0.095 DOM buired half way into the floor pan and stitch welded to the pan.
Using the totally unscientific method of jacking up one corner to change the tire methodology, I got the same observed stiffening results from .120 2x3 square tube installed logitudinal as from an Xed structure of 1.5 x.090 DOM round tubing. Or course the X arrangement did not allow much room for exhaust and may not be practical in a street car.
