Torsional stiffness is calculated by taking the diameter to the fourth power. So diameter is VERY important. A hollow bar which has a slightly larger diameter can be stiffer than a solid bar that has a smaller diameter. Taking something to the fourth power really magnifies the differences. A hollow 7/16 pushrod is a lot stiffer than a solid 3/8 pushrod. I use hollow anti-sway bars in all of my cars. I think I made the first hollow anti-sway bar for a Mopar but I could be wrong. I had one fabricated for my Coronet by a chassis shop back in the early 80's. Nobody offered them for sale until fairly recently and that might have been because I kept bugging people to make them.