Just as a real broad generalization, when I think of A+ schools, I think of teams that make Final Fours; A teams are those that go to Elite 8's. A- are Sweet 16 teams; B+ are 2nd Round teams; B are one and done in the NT, or win some games in the PT. So I don't think LSU being at an A- instead of an A is an egregious failing of the system.
Prestige has to be based on both post-season and regular season; otherwise a team that was ranked in the Top 10 all season, but went out in the first or 2nd round, would have a prestige that was too low, whereas a team like real-life George Mason would suddenly have vaulted up to the A level after their Final 4 appearance, despite being off the radar throughout the entire regular season.
No, recruits don't care about RPI, but the RPI is one way to evaluate the regular season portion of the equation, flawed metric that it is. The other alternative would be to use Top 25 ranking, and there LSU in the four year window was a #10, #19 and two unranked seasons. That does not scream "A" to me, either.
So again, while I respect that people can disagree about A- vs. A, we're splitting hairs over 1/3 of a letter grade, and as such, this just can't be held up as an example of some great failing of the system. I'm not saying things are perfect by any means, and I'm sure there are good examples of prestige being out of whack. I just don't see this as being one of them. And this is not meant in any way as being a knock on mmt, who's a great coach.