A study of a top 25 school showed that legacy admitted students had identical scores than non-legacy: https://hechingerreport.org/proof-points-why-elite-colleges-...

“Among the admitted legacies, grades and test scores were indistinguishable from non-legacy students. Both groups had an average SAT score that surpassed 1430.”

At Harvard, my understanding is that the legacy admits have slightly higher test scores than non-legacy admits.