I haven't heard of studies on this but I would say the effect is potentially huge. For instance, just going by numbers of legacy admits at Harvard: www.cnbc.com/2019/04/07/harvards-freshman-class-is-more-than-one-third-legacy.html

And any natural experiment that could have been done due to JHU's change was probably messed up by the largest donation to a university, ever: www.nytimes.com/2018/11/18/us/michael-bloomberg-johns-hopkins-donation.html