If you have an exceptionally good interview, and the interviewer actively tries to get you accepted, how much of an advantage can that give you?
Not much of one at all. Alumni interviewers just on’t have that much power. We simply add an additional piece to the application. We can’t really “actively” try to get a student accepted except that we can write a glowing report. In our report we can advocate for a student - write how we think the student in perfect for Penn and Penn would be crazy to not admit them but it just doesn’t carry THAT much weight - and I don’t know about other interviewers but I certainly see many fantastic students each cycle. Generally interviews confirm the information already in the application although sometimes, especially with a student who has not had a lot of guidance, we can provide key information missing from an application. I’ve interviewed lots of amazing students who were rejected. I’ve had students ignore emails or stand me up for interviews and get admitted. A good interview is better than a bad one, but neither will keep you out - or get you in - on it’s own.