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I submitted a Why X a couple weeks ago, thinking that I had to get it in before my app went complete. I visited the school this week and was really impressed. They, not having my file at hand, suggested that I submit a "Why X" so it seems it's not too late (app is currently UR).

Is there anything wrong with submitting a short (1 paragraph) postscript to my addendum relating my positive experience on the visit?

Why don't you wait until you hear back from the school? If waitlisted then send it as a LOCI. I don't think that your "addendum" will carry much weight in the admissions decision (other than as a LOCI).

Admissions counselor *seemed* to imply that a Why X about my visit could help my scholarship chances. Not really worried about YP or anything at this point. I don't mind writing it, of course, just don't want it to hurt me.

If they're telling you to write it, write it. Send it with an email that you're attaching an updated version of the essay because you enjoyed your visit so much.