Obviously submitting additional materials beyond what is asked for is a double edged sword. They could love it or they could be annoyed.
But, I get the impression, and alum have told me, that Darden really values its alumni community and their input (every school says this, but whatever), so how about having an alum you know send an additional letter saying "this guy is really committed, I know he would be a great fit, he's told me how much he loves darden, just want to offer an additional endorsement, etc. etc." That way it's not just more of the same from you?