According to an article at Auto Rust Technicians, Torque Boxes and Subframe connectors increase stiffness by 20%.
In my opinion the stiffer the frame structure, the softer the springs can be for a given level of handling. Thus, I plan to not only add sub frame connectors and torque boxes, but to stiffen the front structure I plan to add inner fender braces and a lower radiator support from XV Motor sports.