There are two other issues I see.
1) Joint loosening. If done properly, you can make a pretty hard joint with a jam nut. Make sure there is sufficient thread engagement in the turnbuckle to avoid camming the joint loose. Pinch bolts are ALWAYS soft joints. They rely on the pinch joint not being...
You may want a new twistbeam that doesn't use a torsion bar to gain roll stiffness. Give Benteler Automotive a call. Their beams are typically lighter as well. They use a crushed center profile.
WRT roll center, the Benteler beam may have a problem with it being too HIGH. You will need to...
