It would give you control and prevent her rushing off.....but I have to say that I'm not a major fan of flexi leads in a situation like this.
Have a think about what you are actually teaching the dog by using the flexi lead.
To my mind you are simply teaching her that she can't do what she wants when she is on the flexi lead. Let her off and the problem is still there.

You do have the skills to handle this as you have proved in the ring and at agility. The problem is partly confidence on your part and partly the fact that the control while "training" is lost when "not training". Try to narrow the distinction and raise your expectations. Get the dog working a little harder out of the "training" sessions and get things happening on your terms.