Thats pretty much an unanswerable question given so many variables involved, and if there was a set answer, then a lot of people would do it, thereby nullifying the uniqueness you would gain from doing it. I will have a PhD when applying to med school, relatively speaking, more unique, but how far will that take me? Beats the hell out of me.
But I can tell you this, although the entire applicant as a package is important, extracurriculars rarely make up for low GPA and a low MCAT. The better question would be how low is your GPA and MCAT. EC's and academics are apples and oranges.
For instance at the UC med schools, if you don't meet the GPA/MCAT cutoff, you don't even get a secondary application despite what kinds of EC's you have. Which means the med schools won't see your letters of rec, and there is no chance for even an interview. So yea, at least initially, GPA/MCAT are important, and uniqueness probably plays a bigger role during the post-secondary application phase.