I think it’s highly unlikely that Chicago differentiates among applicants based on whether their SAT scores are above or below the 75 percentile for matriculants (800 CR/790 M). One multiple choice question on a test that maxes out at Algebra 2 just isn’t useful for sorting applicants of this caliber. The most competitive kids stand out in a way that’s more qualitative. Some of that comes down to accomplishments or life experiences. But if you don’t have anything like that going for you, it really come down to crafting an interesting/appealing/memorable application that leads adcoms to think “this is the kind of kid we want” and/or “that kid will love it here.” And talk’s cheap, so show – don’t tell. The Uncommon Essays are there to give them a sense of how applicants’ minds work.
Where legacy fits in is benefit of the doubt re fit/likelihood of accepting the offer (and having a parent who raised you to be the kind of person who would thrive at Chicago).