Hi Keystone,
There are some varying thoughts on what type of cue or shaft should be used for breaking. One thought is, as you metioned, that stiffer yields more power & it may very well do that provided that one can still make very good center contact on the cue ball with one's break stroke. If not the cue ball will deflect/squirt off & result in a non-solid hit of the rack appex ball. Another theory is that by using an LD shaft any mis-hit of the cue ball will be lessoned by the less squirt of the cue ball yield a better hit on the appex ball.
Another debate is heavy vs lighter. I think this, as in most dilemmas, the decision depends on the individual. For any individual, as the weight of the cue is increased, a point will be reached where it is past their limit to propel it at its fastest & thus the cue ball at its fastest. What is that point for you. One needs to experiment with different weight cues to determine what weight is best.
I think the same should be done with stiffness. How well do you strike the cue ball on the break. Very well, more stiffness may be beneficial. If not maybe a shaft with more deflection(LD shaft) might be benefical.
I hope this helps or at least gives you more food for thought.
Regards,