there's not just like, one level of success you need to reach to get an a+. its just a sliding scale for prestige, each season gets scored, and the last 4 seasons combine together (along with baseline and conference prestige) to give you a single prestige score.
certainly, four elite 8s would get you that a+, and i think four s16s would have a shot even, but the rpis and records are part of the prestige score for the season so its not just about the post season. i'm not actually sure what all impacts the season's prestige score but post season and regular season are both significant contributors (record, rpi, post season, are probably the minimum list, i don't think cts and conf championships impact it)