Student learning in instructional physics labs is a growing area of research that includes studies exploring students' beliefs and expectations about experimental physics.  To directly probe students' epistemologies about experimental physics and support broader lab transformation efforts both at the University of Colorado Boulder (CU) and nationally, we developed the Colorado Learning Attitudes about Science Survey for Experimental Physics (E-CLASS).  Previous work focused on establishing the accuracy and clarity of the instrument through student interviews and preliminary testing.  Ongoing validation efforts include establishing the extent to which student epistemologies as measured by E-CLASS align with other measures of student learning outcomes (e.g., course grades).  We find a weak but statistically significant correlation between final course grades and E-CLASS scores from two semesters of upper-division lab courses at CU and no statistically significant correlation for introductory courses.  Here, we discuss implications of these findings for the validity of the E-CLASS instrument.