Don't mind the math major-
11 or 10 isn't enough to be statistically valid in this case. That being said, if one more ACC 900+ stayed, or one more non-ACC 900+ left, we're looking at less than a 5% difference on each category. More valid data would obviously be tracking this over a few seasons to see if that's a problem.
Also, if we're testing the postseason success vs. EE's, shouldn't you break it up further, such as ACC Elite 8 or farther teams, ACC non-Elite 8, Rest of world Elite 8, Rest of world non-Elite 8? The variable being tested isn't being in the ACC, it's postseason success.