I cannot remember seeing any manufacturer to give bending stiffness. I would say those who give are giving bending spring rate, which is bending flexibility. To confuse more there is working spring rate and initial spring rate. Working has to be tested the other can be calculated. Working is lower of the two, maybe some 30%.

Dave mentions correct length is important. The issue is discussed in the documentation. Correct length is the convoluted length and nothing more.

Sometimes it looks like zero length means to some designers very short. Next time when anybody includes compensator into their pipe design make sure you allow sufficient length for it. And do not specify low spring rate, be realistic.