@Mamelot I do know of a bunch of kids who claimed legacy status because their parents went to grad school at UChicago and they got in, so I think grad school legacy did help them but they were also competitive applicants.
I do think a competitive legacy applicant is looked at favorably at Chicago. Legacy may not help a weak applicant gain admission, but in my opinion, it certainly gives competitive applicants an edge, even if it is a small edge. Given the uncertainty of admission at other peer schools schools such a legacy student may find his/her chances better at UChicago
Having said that, I am not sure how not applying ED will affect a legacy student’s chances. That is a new wrinkle this year, so we don’t have any data. Personally I don’t think EA is a very good option for UChicago anymore, but I am speculating without any real data here. In fact I am inclined to think that they will sunset it within a few years