And I understand your point. I'm afraid I am either not being clear or you are not referring to the staircase analogy.
Yes, interview performances are definitely judged without regard to SES, race, or anything other than the actual performance. The point of the staircase analogy is that different levels of interview performance are required to receive an A, depending on what step you are on going into the interview.
A low SES URM is probably on a higher step than a high SES ORM with a given set of stats, all else being equal (and, of course, all else is never equal!
). What this means is that the OP probably doesn't need to be "greater than great" in his interview to score an A, even though standards for performance are the same for everyone.
A 516 is also the same for everyone, but, not really after taking into account the obstacles some people have to overcome to achieve that score as compared to others. That's why some people with 516s receive IIs from T5s while other people with 520+ don't.