Well, the power comes in bursts right? I think the major drawback would probably by the time limit then. It's a power that only works for a very short period of time. She can make her legs faster just long enough to get out of the way,or  she can make her arms stronger just long enough to pull herself over a hurdle. Have it so that she can only augment select parts of her part with one select attribute at a time (I think this was already established) for a short duration, and has to recast the ability every time it wears off. 

Then it doesn't become a matter of conservation, but of precision and timing. It leaves room for her powers to grow, since she's still young, and it fits with her character's role as an acrobat and performer. While she manages to make it look easy, in actuality it takes an intense amount of concentration and effort. Naturally, anyone would become exhausted due to the strain if they were to keep it up for too long.    

That said, I can also totally see Hector and Juliette becoming BFFs. He cooks, she talks. 'Nough said.

Edit: Looking at Rex's post, there's no reason the two can't work in conjunction.