Thats a very good and critical point. My wife pulls 52 pounds at 26 1/2" with a relatively heavy arrow for her set up. The rage 40ke's proved themselves out of her set up on a tank mulie last fall, getting even better penetration than I even expected. I have always been somewhat critical of mechanicals myself until recently. Rear deploying broadheads (specifically rage) have performed very well for me the last 3 years. The ulmer edge broadheads have a design that appears to have some improvements over the rage heads.
We have tried a few fixed broadheads out of her set up, but ultimately her less than perfect form has her arrows flying poorly. She can shoot great groups with field points, but fixed blade heads exaggerate the problem and her groups blow up. We are still working on her form and tuning, but until we exercise those demons, she'll be sticking with the mechanicals.
Hopefully this gives you a better insight into the situation. Also, she has a self imposed limit of 25 yards, so downrange energy is not as much of an issue. (She stalked in and stuck her buck last year at 8 yards.)