Regarding grades among MT auditioners - here is some data on Musical Theatre students who participated in a fairly large regional audition where I worked as a volunteer (there were several dozen students in this group):
GPA 3.5 or above (unweighted) = 50%
SAT/ACT 1800/26 or above = 35%
Students that met both criteria = 27%
Top 10% of class = 32%
Students that met all criteria = 18%
From this data set, high stats students seem to be fairly common among MT auditioners.