what is in the rest of your suspension?
The first question I'd ask.
If you're running rubber instead of poly, start eliminating suspension movement there. Analogy: why put a dead bolt lock on a screen door? Analyze the total sytem you have and address the weak area.
Having Guldstrand's suspenion on my car years ago (Heim joints everywhere), I was amazed at the impovement (control) due to how little the car's geometry changed in corners, even in street driving. A stock ZR-1 is very loose by comparison. While a lot of work to self install, I am glad I did it, for my style of driving.
Seems the C-4 is better known for frame twisting, which the X brace will help. (vettelt193) I have seen little on the cross brace and had read their literature. Whether actual practice supports the general theory is likely best answered by what the racers did with these cars, like Rippie. I am leery of literature spouting general theory and no proof based upon reality.
I don't dispute Gary's comments but wonder if everything else on the cars was identical.