One typically wants to see optimal performance, but without requiring extraordinary effort. But given the choice, I would far rather see peak performance. Most evaluators will preface testing with something along the lines of: "some tasks or items will be easier and some will be harder, and that's expected. Different people will find different parts easier or harder (because we are all unique). Just do the best you can." The rest of the introduction will usually be elaborated more specifically depending on the qualities of the individual being evaluated.
The ability to perform at an age-appropriate level for short periods of time by recruiting other personal strengths is a relevant and important finding, especially when working with a 2e learner, as many settings have difficulty acknowledging the second e because of this. Having that documented in an evaluation is actually quite powerful, as this often is one of the pieces that I find resonates with teachers, when I explain that a student's performance would be expected to be variable, depending on whether they expend that high cost on a particular task. You may have experienced teachers claiming that inconsistencies are due entirely to motivational or volitional reasons because "sometimes they can do it," without recognizing that maintaining that level of effort would be unsustainable for almost anyone.
If she is being evaluated by a competent professional, the effort she puts into optimal performance will be noted and included in the clinical profile. If she is not being evaluated by a competent professional, it won't matter how much effort she puts in...
