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I have a >75th LSAT and <25th GPA at most T14. I'm going to submit an addendum regardless, but does the content/ reason of the addendum really matter? I had an undiagnosed illness for ~3 years, and basically got a 3.8 after getting treatment. I know they won't disregard my gpa because it still impacts their stats obviously, but is there any evidence that suggests that splitters with the same stats and different addendums/ grade trends get treated differently by addcom?

An explanation like that plus your upward grade trend will make a difference. Based solely on numbers on GPA addendum statement, you will likely have a competitive application at the schools where you are a >75/<25 splitter. Hard to say exactly how competitive without knowing your LSAT and GPA numbers and the schools you are looking at